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It sounds like a possible injury to the hind legs or hips, though if she is a older dog arthritis is a possibility. If she is a older dog you can go to your local pet store ad pick up some pain reliever to help until she can see the vet and he can prescribe a inflammation med or a pain reliever.
If this is not a senior dog there is the chance she may have a broken or fractured leg. While there is a chance of it being a pulled muscle or torn ligament you will want to have it xrayed never the less. If it is broken or fractured and you do not seek treatment she runs a high probability of experiencing pain, malformation, and arthritis.
Then I doubt there is a arthritis issue. I suspect she has developed a injury to the hind quarters or a break. this is more common in dogs as they do allot of jumping on and off of high places. My recommendation is to have it examined by your vet and a xray done if needed.