So back when I was 17-18, there was this girl in one of my classes who used to flirt with me, yet she had a boyfriend, she would make eyes and faces at me, this happened as we became friends, but she was the only girl who had ever shown that level of interest in me. I was confused as she had a boyfriend, for some reason, girls with already have boyfriends are more friendly to me than single girls, which I find funny, I know just because a girl is nice doesn't mean she wants you, but this girl was overtly friendly to me, I didn't do anything because I knew her boyfriend and was friends with him, and would never do that to another guy.

Recently she messaged me on Facebook saying happy birthday, which confused me because she was the only girl on my list to do so or bother to do so, I didn't reply back.