Why didn't she just tell me that she had a boyfriend, instead of leaving me confused about what is happening?

I say this because when I met her she was really friendly and flirty. But then next time she was looking really tense, but at the same time she was ignoring me and turning her body away from me. Then when this guy came, she just left with him and she didn't even say anything. It has been like that pretty much for some time.

It just pisses me off that she wouldn't be honest and just say that she had a boyfriend, instead of making me burn my brain trying to figure out what is happening. It could also mean that she doesn't have one and she is just making me jealous, but whatever same thing anyways.