What Causes Difficulty In Raising The Hand?

I Jared my left arm mucking around boxing with my son about six months ago. Felt I did something straight away so tried to not use it as much as possible but is now getting a bit worse . Used to only hurt if I moved it suddenly a certain way but now I m having trouble raising it back to do my bra up as well as leaning straight forward and lifting . And now reaching straight up although that s not too bad. I don t think it s my shoulder more the muscle in top of left arm. Is there some sort of exercise to fix it?