What Causes A Large Bruise On The Upper Back?

I have a degenerative right hip. I fell over the holidays and injured it quite badly. I was given the ok to return to exercise on Wednesday. I returned to the gym today. I ran on the treadmill and then rode the bike for 45 minutes. Tonight My lower right back in the hip bone area was hurting . I have discovered a large bruise about 3 inches wide and 2.5 inches high on my back. I have not bumped into anything or fallen. What could be causing this very large very dark bruise?