I just saw scraches on my girlfriends back.?

The other day I realised that I had never seen her back, she always hid it and at the beach wore t shirts so I asked her about it and she said it was nothing. Today she bend down to get her phone and her shirt went up a bit and I saw loads of scratches and bruises on her back. They weren't like cat scraches but knife and fingernails, Should I ask her about it? What do you think has happend. I want to prepare myself for the answer. Loads of things have gone through my head. She might get really annoyed at me.