Bruising 4 months post surgery

Hi All, has anyone else experienced significant bruising well after surgery?  I had a bilateral mastectomy in May and finished radiotherapy a week ago (yay!) but now have a hell of a bruise on the prophylactic side that didn't get radiotherapy.  I'm confident I didn't punch myself in my sleep so was wondering if it was just one of those new weird and wonderful things that happen to us booby people now?  Thanks :)