[Rivière]

I think from all the evidence you've gathered, it'd be far too much of a coincidence. Sometimes when people have certain reactions over things it can be because they're wanting to cover up for themselves, like your friend who's freaked out by people hurting themselves, maybe she does the same? She might pretend to be freaked out by the idea of self-harm to cover up for the fact she does it herself, that way no one would suspect. I don't think you're overreacting at all, you have every right to feel like you do because you're worried for your friend. If I want to know something from someone but don't want to ask directly, I'll sort of skim around the edges. Sort of like a spiral, asking certain questions around the subject but not directly towards it until I'm met with the information I need to know. With your friend you could raise a conversation about self-harm and ask her about short-sleeved clothing, this way it'd help to reveal if she's been cutting her arms, same with shorts, that way it could help determine if she cuts her legs. Although there are places you obviously can't ask her to show you, at least talking about short-sleeved clothing or wearing shorts can help skim around the idea of self-harm without revealing your intentions.