Why is she acting strange?

So I've been in a relationship for 9 months with this girl who I'm currently trying to break up with I just don't feel the same way. That's backstory now here's the problem/question there's another girl whom I got really close with during my senior year of lacrosse we connected great and we clicked and naturally we developed a flirty harmless relationship we would text and snapchat keep in mind I was in a relationship during all of this. Me and her would have deep conversations and one day she just stop snapping me and texting me. I would snap and text her but I wouldn't get anything back. So I figured she got bored or caught feelings so she backed off. Fast forward to this past month our relationship has re kindled and became more flirtatious that before and sexual through snap. Same thing with the not snapping back is happening again I'm single and I figured she wouldn't pass up the chance but she's acting strange were both in college btw so ladies or gents please a little advice?!

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