My best friend and my partner get along a little too well… and I don’t know if I’m overthinking

I feel weird even writing this. My partner and my best friend get along really well… which I thought was a good thing at first. But lately, I’ve started noticing small things. They text sometimes. Not constantly, but enough. They have inside jokes I’m not always part of. And when we hang out together, I sometimes feel like the third person. Nothing has happened that I can clearly point at and say “this is wrong.” But the vibe feels off. And now I don’t know if I’m just being insecure or if my gut is trying to tell me something. I haven’t brought it up because I don’t want to sound controlling or paranoid. But at the same time, ignoring it is making me more anxious. Has anyone been in something like this? Did it turn out to be nothing… or something you should’ve trusted earlier?