Painful Bump On Arm, Redness

yesterday i was on the computer when i put my arm on the armrest of the chair. once i placed my arm there it hurt as though there was a bruise but when i checked what it was it just looked red. it was a small bump but it hurts like a bruise. i thought nothing of it when today i hit it on accident and it hurt and when i checked again it had a tiny white dot in the center. all around its red. The whole size of it is about the size of a quater.