How do you feel about your S.O. hanging out with someone they used to date?

So my boyfriend tells me he is going to dinner with his friend that is female. I'm not a nosey girlfriend and I trust him, but the closer it was getting to the day, the more insecure I felt. I had never heard of this friend before. The only women he talks about are two that I have met that are in relationships or his best friend's wives. I finally ask how he knows her and tells me they dated in the past. This all occurred yesterday and I'm still uneasy about it. I don't believe he did anything wrong; I more so can't get over how jealous (and other emotions) I feel.