Well, it's a tough group of questions. Have you been a party to any of these conversations? Does your husband talk with his ex in a different way than he does with you, or about things that you two don't talk about?
Do you think you have bad or lacking communication skills?
I keep in touch with some of my ex-girlfriends because we have shared history, friends, and with one we've sort of combined our families in a large extended one, and are more like brother and sister now. None of this would be sufficient reason for suspicion of cheating.
If you are cut out of the conversations, and they try to make sure you're not included, then you may have grounds to worry.