She might be one of those people who are really hard on themselves and can't forgive themselves. Or (I don't know her) she could have blamed herself in the hope that you would comfort and forgive her. Or maybe even a mix of the two.
But either way, cheating doesn't have a very strict definition. There are grey areas and I think my advice would be to put it aside, stop wondering if it was cheating or not, and discuss what would be cheating or not from now on, so that you can be sure you're on the same page now.
I don't think personal opinions are very relevant, but I will share mine for those who are curious: I think if you were hiding something because you thought it was wrong, even if the person would have been fine with it it was a form of cheating. If things happened without you noticing and you were not trying to hide it, though, even if after the fact you feel terrible I don't think it was cheating. Cheating is, in my opinion, something you have to actively do, or actively refuse to prevent.