My 7 year old son has a dark purplish colored raised bump on his right posterior forearm. It s the size of a small marble & is hard to touch. He complains of occasional itch, no pain when i touch it or push on it. This spot appeared right before Christmas, seemingly went away a couple of weeks later, but has is returned. It almost looks like an inflamed pimple or maybe related to an ingrown hair follicle? Any ideas of what it is or how to treat? Thank you!