I am sorry to hear that Bella is carrying her rear leg, has muscle loss, and that her limp seems top worse after lots of activity.
If we see muscle loss then we can be pretty sure this is a long term issue.
Because there is no history of trauma a fracture is unlikely, though possible if she is very fine boned and jumped off a high piece of furniture.
In a small breed dog with rear leg lameness a luxating patella (knee cap that pops in and out of place is a possibility.
With a patellar luxation the kneecap slides out place rather then staying in the patellar groove. When that occurs the leg cannot bend or flex as it should and the dog becomes lame.
This condition is usually congenital, meaning the puppy is born with the genetics to develop the condition as they grow. It doesn't mean that the pup is affected as soon as they are born however, and in most cases it is a condition that we see develop as the bones, ligaments and tendons that form the knee joint grow and develop or in an older pup that gets arthritic. Not all pups in a litter are necessarily affected.
If your girl has this your veterinarian can examine her, and if her knee cap easily pops out of place then a diagnosis is made. I also recommend that they take some radiographs and tell you whether surgery is needed now or she could benefit from joint supplements such as glucosamine/chondroitins and omega 3 fatty acids or a supplement called Duralactin as well as pain medication. I do think that Bella is painful because she is carrying her leg, but her stoic behavior can make that pain hard to diagnose.
If you'd like to read more about pateller luxations here is a link to an excellent article written by the American College of Veterinary Surgeons:
In mild cases some dogs learn to kick their rear leg and pop the knee cap back into place, so the pup may only be intermittently lame. If she isn't lame at the time of her exam, and her kneecap is in place without manipulating her kneecap to see if it is loose the diagnosis may be missed.
Over time however because of the constant wear and rubbing of the knee cap moving in and out of place we see secondary arthritis forming. That makes the leg more likely to be painful after long periods of activity.
Whether she needs surgery or not now will depend upon how severely she is affected. In most cases a dog with patellar luxation does need surgery at some point.
Some dogs will tolerate the arthritic pain and not limp but suddenly become lame if a piece of the arthritic joint breaks off and floats in the joint, causing acute pain.
Treatment will depend upon how severely she is affected by her disease.
It may be a simple as medications to control her pain and lessen her arthritis formation or she may need surgery to keep her comfortable.
I would start with medication to see if we can get her relatively comfortable now.
Long term for any joint pain I recommend using a combination of a glucosamine/chondroitin product (examples are Dasuquin or Cosequin) and an omega 3 fatty acid (like 3V Caps or Derm Caps). I recommend an omega 3 fatty acid dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give her 20mg of EPA per pound of body weight per day. For example a 15 pound dog could take 300mg of EPA per day.
Omega 3's and glucosamine/chondroitins work synergistically and improve cartilage health and joint fluid quality and quantity as well as reducing inflammation. They can take several weeks to see full improvement but some dogs do very well with them alone. They are available over the counter.
Another option is a product called Duralactin. This is an anti-inflammatory product derived from milk proteins and it also has omega 3 fatty acids incorporated into it which can be very helpful. See this link for further information: http://www.duralactin.com/products_canine.htm
If those medications are not enough her veterinarian can prescribe drugs that are more potent. Veterinary drugs we can add include a nonsteroidal like Metacam, Deramaxx, Previcox or Rimadyl. If those aren't enough we can add another drug in the opiod family called Tramadol and/or another drug called Gabapentin.
Another possibility is a partial tear or complete tear of her anterior cruciate ligament in her knee. This ligament crosses the knee and stabilizes the joint. When the ligament tears there is no stability to the knee joint and the dog suddenly becomes leg carrying lame. It can happen suddenly with the smallest slip, especially one that causes a twisting motion to the knee. Given time some scarring occurs so the leg may become usable again, but it is often not as stable as it was, and is painful.
Ideally surgery would be done to give her knee normal stability because without surgery secondary arthritis formation will occur sooner and to a more severe degree then with surgery. It is also more likely that she will rupture the cruciate ligament in her other knee because she will be putting more stress and strain on the other leg.
Your veterinarian can examine her, paying close attention to the ability to manipulate the joint and check "drawer" or forward laxity of the knee. We will often sedate our patients if they are very nervous or painful, or very stoic, to check the knee and take radiographs of the knee joint to look for a characteristic change in the location of the fat pad in the knee joint.
If cruciate surgery is absolutely not in your budget you can try strict rest. Given her smaller size that may be a reasonable plan depending on how painful she is and whether we are able to get her more comfortable with medications and rest. With very strict rest the knee will form scar tissue and gain some stability with time but it won't ever be normal and it will be arthritic. When I say strict rest I mean cage rest, no running, jumping, climbing stairs or playing for at least 6 to 8 weeks. She needs to go outside to eliminate on a leash so she is not overly active.
Surgery or not I also recommend keeping her on the thin side, or weight loss if she is overweight to decrease stress on her knees.
Long term for cruciate injuries I recommend using a combination of a glucosamine/chondroitin product (examples are Dasuquin or Cosequin) and an omega 3 fatty acid (like 3V Caps or Derm Caps). As with the other conditions we are trying to slow arthritis formation.
The other possibility for lameness, though unlikely because her knee seems to be the source of her trouble, is an intervertebral disc disease flare up. This is when the spongy discs between the vertebrae in her spine either prolapse or leak and put pressure on her spinal cord. If the affected disc is in the back this pressure can affect the nerve roots to her rear leg. And if the disc prolapses more on one side then only one leg may be affected. This is quite painful and can lead to painful muscle spasms, loss of muscle mass, or if there is enough pressure then even paralysis can result.
Best of luck with your pup, please let me know if you have any further questions.