Suggest Cure For Bruise On My Forearm

I often get bruises that have no cause, and that are completely painless. I have one now on my forearm that is about the size of a quarter, and in that dark, reddish healing stage. I never hit the area, and the bruise does not hurt at all. even when I press on it, which would be painful for a normal bruise, I can't feel it. I thought it was dye from something at first, but its been days and I wasn't exposed to anything that could stain my skin this color. its been about 4 days.