I've gotten that pain as well. I started on a concert size and eventually switched down to a 3/4 or parlor. Sounds like yours is a dreadnaught and they can be hard to play especially for a beginner. I suggest you go to a guitar shop and try a few smaller ones. Look in the mirror or ask the salesman to see if it's the right size for you and that you dont have your elbow so high. Also review the strumming techniques in fundamentals course. Make sure to stay relaxed. That's key. Good luck.