I Have Sever Pain In My Left Shoulder. Went To

I have sever pain in my left shoulder. Went to my doctor yesterday and he took X-rays, which showed nothing. He wrote me a prescription for Tramadol HCL 50 MG. This has been going on about 3 days now and doesn’t seem to be getting any better. This morning I noticed something like rash with bumps in it on the back of my shoulder which is pretty much centered over the source of the pain. Arm is swollen also. I’m wondering if I could have bit by something? Thanks.