What Causes Lower Back Bruising Persistently?

I am 39 years old just had a recent MRI on my lower lumbar area. Unfortunately I have 3 herniated discs starting at L3 running down to L5/S1. A few weeks back my wife noticed buising on my lower back. We assumed it was from the heating pad but they are still there now after about 3 weeks. My doc was dismissive of them at my last visit, thinking heating pad as well. But the odd part is they are still there 3 weeks later, about the same pattern, and size which is about 6" in diameter right in the center of the small of my back...should I be concerned or is it nothing to worry about, eg just residual heating pad bruising...?