Every time her ex name is mentioned she changes, why?

More details would be helpful. The only guess I can hazard is that there are painful memories being brought up that she wants to leave alone but without knowing more it's a real shot in the dark.

Yes,more info please.

Have you tried asking her?

Are you mentioning the ex? Are you friends with her ex? If not, maybe you shouldn't mention her ex. If he was abusive or just not a good guy to her then obviously it would make her uncomfortable.

Like others have said give us more information, we'd be able to answer you a lot better.

Yes, is there a reason the ex is being mentioned ( kids together perhaps ) normally there is no reason to keep talking about a ex.

Like a transformer? Because that would be cool. But we need more details

If she has a problem with it, is there a reason you continue to do it?

But if you'd be a bit more specific we can give you a more specific answer