Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your girl Lil is subdued and hesitant to jump, and that her stomach seems firmer than usual. She sounds very uncomfortable.
Is she eating, drinking and eliminating normally?
Lethargy can be related to a fever, or stomach upset but if she seems to be eating ok then this may be a pain response, especially as she is refusing to jump.
Dogs that are painful will hesitate jumping, or climbing stairs and display the sort of behavior she is, either because it hurts to do the activity or because they feel weaker than usual. Her firmer than usual stomach can be the result of tense muscles in response to back pain.
I suspect she has back pain given that she is displaying more unspecific symptoms of pain, and that is usually because the pain and weakness can come and go depending upon how she is moving, whereas with a sore joint or leg pain signs and limping will be more specific to one limb.
This may be related to a muscle pull from being overactive or due to a problem with her intervertebral discs, which are the spongy cushions between the individual vertebrae in their back and neck. These spongy discs can move or rupture and place pressure upon the spinal cord which can lead to pain, and in severe cases paralysis.
Radiographs can sometimes be diagnostic but often early on in the disease process, because the discs are soft tissue not bone, everything will look normal. An MRI is the best way of diagnosing disc disease.
If the dog is painful but has no evidence of paralysis we can try strict rest, anti-inflammatories and pain medications for several weeks to allow healing.
If there is evidence or weakness or paralysis then surgery by a board certified veterinary neurologist, as soon as possible, is indicated.
Ideally she should be examined by her veterinarian. If this is indeed a disc problem your veterinarian can prescribe a steroid or nonsteroidal anti-inflammatory to relieve pressure on her spinal cord and nerve roots, as well as something for pain too, such as Tramadol. And if she is having painful muscle spasms then a muscle relaxant such as methocarbamol as well.
She should be closely confined starting now. No stairs, running or jumping. If you have a crate for her I highly recommend using it. The less she moves around the more comfortable she will be and the faster she will heal.
She should go out on a leash to relieve herself. Do not use a collar for her, a harness which more evenly distributes forces if she pulls on her leash is better.
You will need to confine her for several weeks, even as she starts to feel better or she may reinjure herself.
Keeping her on the thin side is recommended to decrease stress on her back, but is no guarantee that she won't have another episode. Once a dog has one bad disc the likelihood of another is very high.
If you are interested in reading more here is a link to an excellent article about intervertebral disc disease, its causes and therapy: http://www.petwave.com/Dogs/Dog-Health-Center/Bone-Joint-Muscle-Disorders/Intervertebral-Disk-Disease/Symptoms.aspx
There are other less common causes of back pain such as infections, tumors of the vertebrae or the spinal cord itself or fibrocartilagenous emboli but far and away disc disease is the most common cause of back pain in dogs.
You can apply alternating hot and cold packs on her lower back for 10 minutes at a time several times a day over the next few days. Cold reduces inflammation and hot packs stop painful muscle spasms that are commonly seen with disc disease.
Elevate her food and water bowls to neck height to decrease stretching and bending to eat and drink.
Then keep her very quiet, no ball playing or jumping for the next few weeks.
Please let me know if you have any further questions.