What Causes Shoulder Pain After A Session Of Pitching?

I'm a pitcher and I've been throwing for about 6-7 weeks now. In an outing last week I threw about 30 pitches and didnt seem to have a problem. At the end of the day I ended up throwing a ball as hard and far as I could. Still I didn't have much pain. Since then my shoulder has been bugging me. It is sort of in the front of my shoulder. I get the pain when I throw, and when I lift my arm to be above my shoulder height. I'm taking today and tomorow off for sure, complete rest, ice and advil. What should I do and what's wrong with me?