If they have children together, I guess that would be alright, if they are text messages regarding the children, or child support. Or if she is still friends with her ex, I don't see the harm in it. If you have a problem with it, talk to her about it, tell her it bothers you to a certain level, but don't seem too demanding about the situation, let her explain to you their relationship, or the reason for the text messages. Also this shows a sign of insecurity on your part, if you trust her, then you shouldn't worry to much about it. Best regards