Girls, does she like me?

There's a girl I work with, I don't know her well, haven't talked to her much, known her for a few months. Last week was my birthday, she walked up to me and said happy belated birthday, what did you do and etc. I kind of admit she seems nice and she's pretty. But when I tried to talk to her today she wasn't as receptive with the small talk. I usually have a rule which is don't pursue girls in the work place. I know this probably doesn't give much but I figured if someone chats with you and is nice overall but I don't want to mistake it for just friendliness.
Girls, does she like me?
Post Opinion