Monkey said:
QFT.
She's basically testing you and if you fail the test she will make it out when she dumps you that she only always viewed you as a friend and that your relationship was a mistake lol.
To win this one absolutely you have to dump her now, dump her first and then you have the power and if you want her in the future she may be yours again.
To win this one and possibly keep her if that is your goal just ignore her talking about this other guy and maybe start bringing up hot girls you see whether they are real or made up by you doesn't matter.
You have to match her tension for your own tension and what that means is don't let her think she has you locked down.
She has to think you are a desireable man to other women and that if she acts up you'll have no problem whatsoever starting off with someone new.
Sadly when you don't do something like this in a relationship the chick can start to get bored and then her eye begins to wander to that perceived big better deal.