Don't know what's wrong with my horse?

He might have strained one of his muscles up on the inside of his front legs (sorry im dont know names of them ><) from stretching out too much by maybe falling over in the paddock or getting into an awkward position. When my horse was very young and i first got him he did something to his leg and this was the problem. It was very suttle and hard to notice but when we did we got the vet out and he gave us some medication and we had to rest him for 6 weeks but the vet even had trouble finding the problem. My horse had the same symptoms of yours (being off at the trot, especially on one side, not letting me pick up one of his feet and being alot slower and stiff in his movements). hope this helps, good luck