Why does he keep talking to her..?

My husband has this woman friend that he talks to. He was friends with benefits with her right before we got together. They're great friends and can talk so easily together. I'll read their messages sometimes and wish we could have our marriage that light hearted. We try but there's always something that needs to be tended to. That we need to talk about. I trust him. I trust her. She used to be my friend until about 6 months ago I found out about them sleeping together before us. I don't talk to anyone I slept with.

Is it ok for me have this feeling? I know they won't have sex ever again. Why do I feel so jealous? And I'll ask him to please stop talking to her and he will, at least for a month or so.

Edit: We're not friends anymore because I asked her before if she slept with him before us and she told me no. There was no reason she should have lied to me. Me and my husband also split for a couple weeks after I found out because he lied as well.

Update: Thank you guys for being so supportive. We had a huge fight and both agreed to delete all the people off Facebook and our phones that we've dated or slept with. I guess he wanted to be a smartass 😂😂 I've only slept with two of the guys I've dated but dated quite a few I still randomly text one I dated back in highschool but we didn't do anything sexual. So that's that. Maybe we have this fight again because he let me watch him block her. So if she turns up again, I'll know what's going on. I told him I'd leave if it happened again. (First time I've ever threatened that and his face was like so sad. I almost had to take it back but I stood my ground ladies.)