Strange behaviour from a girl I once spoke to on a night out

So about a year ago or so, I was talking to a girl while having a cigarette outside a bar, after she came over to me and struck up a conversation with me. She seemed interested, I wasn't sure though, but either way I was a little shy and didn't see her again that night.

Fast forward much later and small world that this is, I end up playing a gig on the same bill as her now boyfriend's band. This is a gig which I've invited a girl I'm trying to get in with to. Also I have reason to believe that she was in a relationship with this current boyfriend when she seemingly approached me that night.

At one point outside she comes over to me and tells me that she remembers me, and I remember her. She starts telling me how her boyfriend is the best thing to ever happen to her, that he's so talented and lovely, and that they've even discussed marriage a few times. Then, she starts firing questions at me about this other girl. How did I meet her? What's she like? Am I into her? How long have I known her?

Whenever I'm chatting with this other girl throughout the night and we're laughing or whatever, and the first girl's in my sight, she keeps glancing over at us and at times seemingly looks a little displeased. Also, at other times, whenever I'm in her sight, all of a sudden she starts kissing her boyfriend, or cuddling him, or sitting on his knee.

Any ideas what this is about?