Is it okay that she slept with this guy while we were "talking" but weren't official? Am I being unreasonable?

This could potentially get very long, so I'll give the background info in bullets:

-I'm in month two of a distance relationship. Only one state away, so we still see each other every weekend.
-The relationship is great in every way except for the issue I'm bringing up here. I'm seriously in love with this girl (I'm also in my mid-20's and had a 3-year relationship previously, not tossing the L word around lightly here).
-We've known each other for a long time. Hooked up once in college, both got into other serious relationships, and met up again through a mutual friend this summer. Hooked up in July, talked for a while, and became official in late August.
-She broke up with her long-term boyfriend in May. Not a great relationship. He cheated, belittled her, compared her unfavorably with other girls. After that, she started sleeping with one of her guy friends, who we'll call John, as a rebound thing. Eventually he got moody and obsessive/stalkerish when he realized he wanted a relationship and she didn't. When she and I first became official, I was under the impression she and John had stopped sleeping together well before our hook-up in July.

The issue: I recently found out, via drunken confession, that she and John slept together a few more times during the period when we were "talking"--after we'd hooked up but before we became official. She said she never liked him and felt bad for leading him on, but their hooking up "just kept happening," like it was something she had no control over. She also asked me not to judge her based on her actions during that time period, as she was still screwed up over her bad long-term relationship from before.

One week in particular they went away to a relaxation camp thing. It was a week she had a choice of going to that or coming down to see me, but she and John had already paid the deposit on the camp. She didn't want anything to do with them at this point because he was getting obsessive, threatening to hurt himself if she wouldn't be his girlfriend, etc (he's young, only 18). I knew they went to the camp together and I understood since she'd already paid the deposit. But I just found out the part about them sleeping together. She said it was out of pity/she couldn't really explain it.

I don't know how to handle this. Like I said, I'm seriously in love with her and everything else is great. I still feel like I can trust her and want to believe her when she says she was still just screwed up from her ex. This is a stupid thing to break up over. At the same time, I don't think I ever would've started this relationship in the first place if I'd found out about her sleeping with John on the vacation. So I guess I feel like it's bad enough that I wouldn't have gotten involved with her had I known, but now that I am, knowing upsets me but isn't enough to make me break up with her. I also feel like I'm being unreasonable or jealous, but she says I'm not and feels really bad.

Not so much a question as a request for some outside perspectives on the situation. Sorry it's so long, and thanks to any who read and respond.
By vwiz 8 years ago :: Dating
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