My FWB [M25] is meeting other women and it makes me feel uneasy without reason

We have been FWB for 8 months. Neither of us wants anything serious and we are both allowed to be with other people. Yesterday I found out he was on a date with a woman from his home town. He said they didn't sleep together but they were having another date soon.

My heart sank and I began to feel really uneasy and kind of overprotective. I have no reason to react like this but I still feel like shit when he is with someone else. Whenever I'm with him I ask him about his plans and who he is meeting. I always try to steer him to spend more time with me. I managed to get him to cancel a date and come over to my place to watch movies, talk and have sex.

I'm also seeing someone else, I've been on 2 dates with a guy but I'm not sure it will develop to something other than a one night stand.

I don't know why I'm acting like this or why it makes me feel really bad when he is with someone else. I have no problems with it since we are FWB and nothing serious so what can it be?

Tldr: when my FWB meets someone else I feel like shit although I have no reason to