My dc (10) has a small lump on the back of their upper arm. About two weeks ago the area was bleeding when they came out of the swimming pool after a lesson - they hadn't noticed bumping it/catching it on anything. We cleaned it up and didn't think anything more of it. Today, they've said there is a bump there - it looks almost spherical, about 5mm diameter and looks like a blood blister. It seems such a random place to get a blister, the only thing I can think that might have caused it is the straps on their backpack when they put it on/take it off, but even that seems excessive. Is it just a blister and will go away eventually (once we work out what has caused it and stop it)? Could it be a wart?