What causing the pain in my shoulder?!

i went out 2 weeks ago on a girls night out one of my friends was drunk pulled my left arm very hard because she nearly fell over n now its in pain right in the joint, turning my arm in certain directions it felt like something was trapped in the joint and hurts like hell. Its now progressed to being a generalised dull pain that covers my entire shoulder joint, and goes down as far as my shoulder blade and up into my neck. I get the odd pain in the upper muscle of my arm and even a shooting pain in my forearm.its even worse at night the pain gets bad n i cant sleep..