When your girlfriend ex askes her the question me or him what does that mean?

It sounds like he's giving her an ultimatum of either being with him or you.

Has she been seeing him while you were dating?

Sounds like she doesn't really trust you.

What that sounds like is that there is a possibility that your girlfriend and her ex have been in contact lately. Why? Because he is giving her an ultimatum, and problem is unless the guy is dumb he believe that he has some power over your girlfriend or at least enough for him to convince her to drop you and go back with him. Personally, tell your girlfriend about this and see how she feels, if she is planning on going back then tell her to stop teasing and leading you on and demand a little respect as the boyfriend that you are. If she doesn't, then ask her to stop seeing that guy, what is the point, no benefits come from the current friendship that she has with him regarding your relationship. Do not accuse her of anything when talking to her about this, it is only going to create stress between both of you, so stay calm and keep your cool.

Good Luck,

Javi