It sounds like she was trying to figure out what her status was with her boyfriend. You just happened to be there when they were going through some troubles. She saw you as a potential romantic partner (best case scenario) or she was just using you to make her boyfriend jealous (worst case scenario).
I wouldn't mention the ex/not-ex to her at all. If you want to ask why she deleted you off FB, go ahead, but be prepared for whatever answer she might give. Maybe her boyfriend doesn't want her talking to you or something.
I would just play it cool and not really consider this girl to be a potential anything for quite a while. She is a neighbor and fellow student only at this point, so be cool and treat her accordingly.
Good luck.
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