Do you remember hurting it? Maybe you have pulled the muscle somehow. It could be a ganglion(sp)...I have one in my left wrist that joins my hand to my arm. It became a lot worse when I was pregnant, which is common. I didn't have a lump their for a while and then it just came up. It has gone back down again and doesn't hurt anymore so dunno whats happened there. Maybe just ask your GP about it.