Whats wrong with my Dog?

My soon-to-be seven year old Labrador has had a strange limp for a couple of months. When he gets up from laying down he can't put put pressure on his rear left leg and hobbles along for about 1-2 minutes, and can then walk, run and jump as if there was no problem. I also note that at the top of the inside of his thigh there's a lump of (what I believe is) muscle. It looks as though he's torn a muscle or tendon loose, or possibly tore a muscle. My vet originally said it was arthritis and recommended supplements, they have not helped. Would such an injury cause him to walk this way? Could it be something else? The dog does not appear to be in any pain other than his reluctance to use his left hind leg when getting up. What kind of treatments are available?