Well the way i would look at it is this; If she did SI, don't you think she would go more out of her way to cover it up? She would probably be wearing the super-long sleeved shirts, or at least not put an obvious bandage over it on an open arm. She wouldn't be leaving it in the open to question, if she didn't feel comfortable justifying an answer to people.
But if it's really bothering you, then maybe ask her what happened, so that way you can move past this, because I don't think she would want you fussing over her arm - especially if it's causing you this much trouble.