By the way, it's not painful at all.

Hard to say, but something that sounds similar used to happen to me years and years ago when I used to masterbate a lot. The lump was not always in the same place, but was always where pressure had been from masterbating. It went away after about two or three weeks. Have not thought about this recently, but it did occur on about twenty seperate occasions. It was obviously related to masterbation and the points of pressure from the hand. Does any of this seem familiar? If so, try lubracant.