The #1 injury to a hind leg is a partial or complete tear of the cruciate ligament. If the x-rays didn't include the stifle (knee) that must be done next. In addition, vets in a hurry or not paying careful attention will skip doing a very simple test to help rule-out a cruciate tear.
This test is called a cranial drawer. I would ask your vet if they did it. If they say they did, and it was normal, find another vet and confirm it. If they didn't do it, ask for a courtesy exam because it should have been done.
Absolutely, it is time to try a good non-steroidal anti-inflammatory. Rimadyl, Deramaxx or Metacam will do. In addition, I would ask your vet for a pain medication called Tramadol to complement one of the other drugs.
It will be trial and error. Let's see what the medications can do to help the comfort of that leg.
So, to summarize:
A follow up with your vet if they ever properly did a cranial drawer test (a simple manipulation of the leg) to rule-out a cruciate ligament tear.
X-rays. Make sure knee x-rays were taken. Not to look for fractures, but to look for fluid accumulation in the knee joint.
and the trial with the medications mentioned.
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Thanks. Dr. Andy