Hold on...most of the replies is basically "it's nothing..." and it probably is, but there's a very thin line between thinking about an ex and having feelings that the person doesn't even know herself/himself...
Most of the situations it's "yeah, it was only my first love...blablabla.End" and some it's "I hate my first love, he's so stupid for doing those things to me"...this second way of thought for the first love or even an ex- is actually because they're still in love with them...some signs of love come as jealousy, anger, etc...
I'm not trying to scare you, but believe me, I've just lost MY first love because after 7 months she dumped me and tried to return with HER first love, and she said whilst we were together from time to time that he was a jerk...
Anyway, if she talks about her first love most of the time or at least three times a week to you or a close friend, than I would suggest to leave her because she still feels something for him not because it's her first love, because if it was just her first love, she should learn to keep those memories to herself...because to be honest, a first love break-up...should take a hell of a lot of time to heal, I mean 4-5 months at least.
There is a sign to see if all what I said was false and it's simple : see if she still talks to him or still has him on her contacts via e-mail, msn, etc... anysort of communication together, because the first thing to do when you break-up with a first love is to Cut all communications...that's it. If she hasn't done it yet, then she is afraid to leave what she had with him in the past...in otherwords, she can't see it or feel it, but there is a piece of her who doesn't want to let him go...
I hope that I haven't been harsh, and believe me everyone when I say that if I didn't fall for my ex (my first love) then I would probably be saying a whole different story, because like I said mine didn't cut off communication with him, and in the end, she dumped me and several weeks later she found out that she still loved her first love...pfff...
It all depends if we have a strong self-esteem to say "It's over. Full stop."
if we don't have a strong self-esteem and confidence in ourselves and in the choices we make, then most of the time, we'll find our way back to their doorsteps' not knowing why...
Good luck,
Pizzadude