I had a minor operation on my right biceps about ten years ago. Every time I worked it hard, I'd feel like it was bruised for a week afterward. I'm accustomed to muscle soreness, but this was different. I started to notice a little discoloration on the inside of my arm, like an actual bruise. It was some kind of vascular problem, and I'm so glad that I went to the doctor about it instead of suffering, assuming it would get better with time.
Your case is probably nothing like mine, but you never know. A doctor might know, though. Put down your cake, turn off the Martika music and make an appointment.