Yeah, I'd probably be a bit ticked off too. While she has a right to talk to her ex if she needs to, it's rude not to at least give you an explanation of why she has to stop talking to you. If they have unresolved issues, you have a right to know. Just ask her what was so important that she had to abandon your conversation for one with her ex. If she can't come up with a good excuse, ask her just exactly what's going on between the two of them; she ought to get the picture. She needs to move on in her life, leave her ex behind and concentrate on who she's with now; if she can't do that yet then she has an obligation to explain herself.