My girlfriend [21f] distances herself from me [25m] when we're out with friends.

EDIT: I really appreciate everyone's responses and its made me realize this isn't that big of a deal. The only thing I'd like to address is the common perception that I don't have friends, want her to be all over me in public, don't like her to socialize, or have problems socializing myself. None of these things are quite accurate. I think it boils down to me feeling like there is an imbalance when we're hanging out, and its something I believe we can work on together.

Is this a common thing?

Dating 4 months. Whether we're with a group of her friends or on a double date with another couple, my girlfriend shows the absolutely minimal amount of attention she can without being overtly rude, i.e. she won't flat out ignore my attempts at conversing with her but its obvious shes trying to end it as fast as possible and focus her attention elsewhere.

We talked about it, and I asked her if she was embarrassed of me or if it would be the same with any guy and she said that its just part of her personalty to not want to show affection towards me in public. She doesn't just mean PDA, that doesn't bother me as much. But she admitted to, in my words, putting me on the backburner when we're out with other people.

When we're with a group of her friends (I don't have as many friends and they live very busy lives so we're never with a group of them) I feel like an absolute outsider. She has never once, not once, tried to bring me into a conversation, like "oh yeah lazerface is interested in that stuff! Hey babe tell about _____). I start ones on my own (I'm not socially awkward or anything) but it still feels really bad. When we're on a double I honestly feel like that old married couple who obviously don't like each other and just ignore each other the whole night, and it sucks.

Privately, or on a date she has no problem with reasonable PDA or being affectionate towards me. And otherwise our relationship is pretty good, but this is something that really bothers me because it makes me not want to hang out with her friends or go on double dates, and I know that would be toxic to the relationship to basically make her choose between me and her friends.

Does anyone have any experience with an SO who distances themselves from you when you're out with a group?

tl;dr: Girlfriend distances herself from me when we're with other people, not sure if this is common or what to do about it.