Yeah, I actually had a number of those as I was growing up. If I remember right when I asked the doctor, he said it was just a cist - a calcium deposit on the bone or something like that - and it'll go away in time. Sure enough, it did. I have gotten several in other parts of my body since then, but it always comes in the form of a hard lump on a bone and it always has gone away. If it is indeed calcium, maybe I should consider not drinking as much milk
Mine were always on the bone, though, so I'm not sure if this is the same thing as what you're experiencing on your muscle.