How should i react if my fiance tells an ex flame hey sexy?

We were in a relationship for three and a half years, we broke up for two yrs and got back together and are now engaged. While we had broken up he dated alot, we even tried to get back together a couple times but it just didn't work out. He had dated her for a short time but they didn't really last and then we got back together and he said they don't talk anymore but it was her birthday the other day and he told her "hey sexy happy bday" and it just had me thinking that the comment was a bit inappropriate. Am i being paranoid? The thing is she is a model and is in alot of social circles.