silly - she was fucking the guy before she broke up with you, but its been 3 weeks, so she stopped trying to hide it
This is what I was gonna say. Most likely scenario, almost 100%. At the very least she was emotionally cheating, but most likely there was more than that going on.
I've been in relationships that I emotionally detached from before the breakup and I moved on fairly quickly afterwards....but not that quick, and not to the point of having sleepovers, except in ONE instance where I was being pretty sketchy. I realize it goes against the flow for girls here to admit we are not perfect exceptions to every rule haha, but I was a stupid kid and this is one of my least proud moments, but it's definitely something I learned from.
I didn't physically cheat, but I had basically fallen in love with someone else, and he found out about it before I had a chance to break it off. Actually, that's horseshit...the order should have been, end things, meet someone else, fall in love. I make ZERO excuses for the way that went down, and whatever reasons I had for falling out of love with him at the time, he deserved better. Guy has every reason to hate my guts but for some reason (he insisted we try to resume a friendship that I still feel unworthy of) we're friendly and still occasionally "argue" sports via facebook lol. It's one of the reasons I am so vehement about how easy it is to avoid emotional cheating...I can look back and point to a million things I did to find myself in that situation that could have easily been avoided. No escuses for me, no excuses for anyone else.
Bottom line, don't stress it. My ex was better off without me when i was at that level of emotional immaturity (I was 19), and you're better off without her.