What Is The Cause And Treatment For Pain In Upper Arm?

I have a sore upper right arm. I ve had this for about three months and assumed I must have just strained or overused it. However, it doesn t seem to be getting any better. It also looks swollen compared to the other. I also have a cyst in my right arm pit, which I ve been to the doctors about and they said its best left alone if it isn t bothering me. Could the two be related? I ve tried cold packs on my arm to reduce the swelling but it still remains really sore.