girlfriend troubles... help!?

This girl and I have been dating for almost a month now... but for the past week or so my girlfriend (we're both girls) has been acting really funny... and not "ha ha" funny... but she has been less talkative and seems to not want to hangout,,, this morning at school, she told my friend she was leaving the smokers pit before i came out, like trying to avoid me. I didn't bring that up to her cause im too scared to. But when i saw her later in the day she said she had just gotten to school, which was an apparent lie?

So i pulled her aside and asked her if there was anything wrong... anything at all...

and she said no, and that she was just very tired and that she was not feeling good.

but im just not sure if that is the case though... I think she may still like her ex... even though they broke up 3 months ago? she has talked about him quite a few times, so it makes me a bit paranoid i guess (kind of annoying, lol) ... I really like this girl a lot, and don't want to lose her... I have no clue what to do... I've never been in a relationship with a girl before this one. So can anyone maybe help me out a bit? any experienced lesbians? lol

and i highly doubt she's cheating