SWEET, LOVELY JENNY? IMPOSSIBLE!!!

So I need some advice because I don't know what to do.
There is a girl named Jenny that's in a few of my classes. She's really nice and pretty and liked by a majority of the school. I'm not close with her, but I talk with her during class and stuff.
So yesterday in math class, we're all talking together during a class party and I was admiring some of Jenny's nice bracelets. And something catches my eye. Yeah, I bet you know where this is going.
She's got scratches all down her wrist, at least 3 inches down. And they are just scabbing up and healing. So I ask her, I say "Hey Jenny, what happened to your wrist?" And she looks at me and says, ".....nothing." Obviously, that's not true, so I ask her "Do you have a cat that did that?" and she just says "no" and keeps talking to everyone else.
I'm not 100% sure, but what else can I think. No, I don't think cat did that to her. they were too clean and too even and there were too many of them. But I don't want to think that Jenny was doing that to herself. SWEET, LOVELY JENNY? IMPOSSIBLE!!! She's too smart, too bubbly and too happy to do that. And I'm not saying she can't be depressed, she can. I'm happy and bubbly and I'm chronically depressed. But self harm? That's just not her. But what if it is?
So I need to know. How can I ask if she's okay? I'm not that close to her and she may think that I'm being nosy and butting into things that don't concern me, but I care if she's hurting herself. I'm sure her closer friends know and are helping her but how can I sit back and do nothing if she is doing it? It kills me inside. So how do I go about asking her? Please, I'm really concerned.
~Adrienne