Suggest Treatment For Upper Arm Pain

I carried a box in from my car yesterday, up to my apartment on the third floor. It only weighed about 20 pounds. A few minutes later my upper arm started to hurt really bad. If I m standing up with my arm hanging it doesn t hurt, but if I do things like turn the lock on my door or move my arm a certain way, it really hurts. This happens a lot to me just doing everyday things. Any time I ask my doctor he just tells me to go to physical therapy. Could I have some kind of muscle disease? I forgot to add that I m 47 years old and male.