Hello, I am very sorry to hear about your girl.
Because she was able to "warm out" of her stiffness and her age I would be highly suspicious of arthritis and the secondary muscle loss and weakness that comes along with it.
Has she ever had radiographs of her spine or hips?
If you pinch her toes on her rear feet does she feel it?
If you support her standing and flip her rear feet so the topside is down does
she immediately right them?
It is important to find out whether she is too weak and painful to get up or whether she has lost the ability to do so.
It is very possible that she is dysplastic. Sometimes it is simply too painful to get up and go out. Symptoms can happen suddenly if a piece of the arthritic changes in her hip breaks off and is free in the joint.
But if she is dragging her toes when she walks that can signify neurologic problems, such as an intervertebral disc(s) (cushions between the bony vertebrae) that are out of place or spinal arthritis putting pressure on the spinal cord or even a mass in
or around the spinal cord. These seem less likely as she is able to move well now.
Another possibility is a condition called FCE, fibrocatilagenous emboli, where a chunk of cartilage breaks off and lodges in the blood vessels that supply the spinal nerve roots. It is very painful initially as blood supply to tissue is blocked off. The pain only lasts a short time, less than a few hours to a day, but the weakness from the nerve damage it causes it can last for weeks or in rare cases is permanent. Again this seems less likely because she has regained her ability to move rather quickly.
Another possibility if she seems not painful is a condition called ascending myelopathy. This is a progressive degeneration of the spinal nerves that begins with incoordination of the rear legs then progresses to loss of urine and stool control (continence). This seems very unlikely with her as her symptoms came on very suddenly according to your history.
She really needs a veterinary examination as soon as possible.
Radiographs to look for a collapsed disc space or arthritis of the spine and hip dysplasia would be helpful. We need to know what the problem is to treat it
If those look fine then an MRI of her spinal cord in the back of the body will be helpful.
Even if you choose not to have diagnostics done ideally she would see her veterinarian because the prescription medication your veterinarian has for pain will be much safer and work better than any over the counter medications that we take. In fact acetaminophen and ibuprofen aren't used in dogs because their effective doses are very close to a toxic dose in dogs.
The only over the counter anti-inflammatory that can be used in dogs is buffered, enteric coated aspirin (like ascriptin). Aspirin does cause stomach and intestinal irritation and ulceration as well as clotting problems so should not be given for more than 2 to 3 days consecutively and should always be given with a meal. If you choose to use it watch for lack of appetite, vomiting, blood in the stools or dark tarry stools and stop immediately if you see those. Do not use aspirin if your dog has liver or kidney disease or a history of a sensitive stomach or clotting problems.
The dose for aspirin is 5mg per pound orally every 12 hours (about one 325mg tablet per 60 to 65 pound dog every 12 hours). Always give with a meal. Do not use for more than 2 or 3 days.
Be aware if you choose to use aspirin and it doesn't help your veterinarian will be limited on what they can give as there must be a 5 to 7 day washout period between different nonsteroidals or nonsteroidals and steroids.
You can try alternating warm and cold packs on her painful areas for 10 minutes at a time several times a day.
Make sure to rest her, no running , stairs or jumping.
Long term for joint pain I do 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). These 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 that's not enough she should see her veterinarian for prescription 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. These drugs are much safer and more effective than aspirin. Aspirin used for any length of time will create gastrointestinal ulcers and clotting problems.
Best of luck with your girl, please let me know if you have any further questions.