Absolutely no idea. She was screaming her head off all morning yesterday and that's when we noticed bruising. My best guess is acting like an action man, she's learning to walk just now and has been constantly pulling herself up onto her feet, fall on her backside etc
We now have try to immobilise her arm and stop her moving it around for the next 2/3 weeks, which will be murder trying to get her to, but she has been given a wee sling type thing.
Getting her checked for something like that had me feeling very guilty. Each time we were asked questions "How did it happen? When did you notice it?" I found myself getting defensive. "I didn't do this. It was an accident."