Bump on my White's back

I recently bought a young White's (2 inches) and he has a bump on his back about the size of his eye. It's oval shaped, vertically orientated in the middle of his back. It looks very much like a blister except there is NO discoloration of any kind. It is semi-hard to the touch but if you move the skin around the bump it moves as well. Therefore it's not muscular... I don't think. It seems to be a more sensitve area to him when touched. He had the bump when I got him only a few days ago, but it's been so difficult finding a store near me that sells them I bought him anyway. I had Whites in college, but I don't remember them being SO inactive. People on this board are posting, saying their whites are "active." What exactly do you mean? What do they do? Mine sits in the same place till I come back and move him. He looks healthy otherwise and seems to be eating. One of the frogs in his tank at the store had severe discoloration and when i went back the next day he was dead. Red Leg has been suggested as a possible problem with him. Any ideas?

Mark