What Causes Popping Sound In The Arm?

In July, I was a staff member at a week of camp. I was helping to coach swimming. On Monday, my arm started hurting when I went to get in the pool. Whenever I would try to raise my arm, it felt like the muscles around my shoulder were not right somehow. I thought it was just a muscle cramp at first, so I went ahead and continued swimming but it was worse when I got out of the pool. It got to the point that I couldn't even move my arm without being in pain, and the camp nurse told me that it was most likely a pulled muscle. She gave me ibuprofen and an ice pack on Monday night, and on Tuesday morning, my arm felt relatively back to normal. Problem is, I still get pains every once in a while when I move my arm, although it's not too severe. Also, when I move my arm in a circle, I keep hearing a popping sound. What's going on? Is this a muscle problem, or a joint problem? And what should I do about it?