I Have Had A Pain In Right Arm From Shoulder

I have had a pain in right arm from shoulder to wrist. Arm is very weak. I went to chiropractor and had therapy for several weeks. It is much better I have not been treated for about month. Much better but now I have a hard knot on top of upper arm I exercise and thought it was just muscle but it has gotten bigger suddenly and skin seems bruised. I am going to try to get into my doctor tomorrow