Abby
Has your vet checked for lyme disease or leptospirosis? Both of these, one bourne by deer ticks, the other by drinking stagnant water, can create lame-ness, stiffness, sore-ness, paralysis, etc. Our boxer baby, Amiga, was diagnosed with both of these diseases two years ago. Antibiotics helped to clear, although we get her tested regularly and sometimes have to put her back on antibiotics and anti-inflammatory pills. She does get lame every now and then in just one leg. It's worth a check and some tests with your vet.