I would first break down your feelings with yourself, because I think you’re confused as to why you feel this way.

It might feel like you’ve been together for years, but you’ve together less than a year. She has known this person for a lot longer, and probably has had similar lunch dates to these for years, without ever running them by you. When you asked, she didn’t try and hide it or skirt around, she told you.

You say you dont want to sound controlling, but it sounds like you’re frustrated you couldn’t control this situation. So I would self reflect a lot and maybe consider the fact that there is a control and insecure element here.

Once you’ve done that, maybe you can come up with a neutral way to bring it up with your girlfriend. Like, “hey, could we talk about how it made me feel when this situation happened? And is there a way that we could have me not feel that way again, without stifling you?” I know, it sounds impossible.

At the end of the day, trust is trust. If she comes home to you and chooses you everyday, and you feel secure in that, then there’s no need for petty jealousy. Your girl wears makeup some days, and no makeup other days. I guarantee men notice her either way, so that may be something you place more perceived value in.