What Causes Severe Pain And Bruising On The Upper Arm?

Hi, A Few days ago I found my upper left arm very painful, very randomly without injuring myself, or even banging my arm. The next day I discovered yellow bruising which has slowly turned into a rather large red/purple mark which looks like blood surfacing to the skin. This has now spread therefore is now a little bigger. This worries me as it has come out of nowhere. Also when i first noticed the bruising my arm was very painful to move, and had slight swelling. Today, the marks are still there, although the pain has moved down my arm and I now have pain in my lower arm, as well as my wrist.
PLEASE HELP!!!