My 10 year old son developed a little red pimple on his upper arm. It is on the outside of his upper arm. I was drying him with a bath towel one night and it popped. It was bubbling out blood for over an hour before it stopped. It popped open another time or two since, bubbling out like a bubbling blood stream, and has now refilled with blood to a larger size pimple. Like a 1/2 of a small pea. What the heck is it???