Lately, my girlfriend has been getting angry at me for tiny, petty things, like not replying fast enough, and then refusing to talk to me for hours over such meaningless squabbles. Anyway, she’s also been hanging out with this guy whom I’m pretty sure fancies her quite a lot lately, in fact, until very early hours this weekend. Both Friday and Saturday night she promised to come see me after, but bailed both times. They’re friends because they’re both vegan, which is another thing she often criticises me for, and sometimes threatens to leave me because of.
I’m normally cool about all this, cos, you know, it’s part of being in a relationship (right?) and you need to be patient, but, just now, this guy posted a pic of just him and her together in a toilet mirror (?!) with the caption “Fav <3”, and it makes me very uncomfortable he’s putting hearts, with pictures of just him and her.
Maybe this is just my impulses and I’ll get over it, but is this a valid reason to be annoyed?
LemonVibe 
I think you have a very valid reason to be annoyed. It’s not right for her to behave like this with someone else if she is supposed to be YOUR girlfriend. I would confront her about it, just explain how you feel and how this bothers you. If she has a problem with it, it may be time to decide if it is worth staying with her or not.
A major of being in a relationship is accepting or at least respecting the other person for who he/she is. If she judges, insults you and blackmails you with threats to dump you over your lifestyle choice to not be a vegan, then she is not a good match for you.
It’s unfortunate that she chooses not to straightforward and honest with you about her doubts about her compatibility with you, instead of driving you away by picking fights, breaking dates, giving ultimatums, spending late hours with another guy, and taking bathroom selfies with another guy and saying nothing when he posts the pics with a heart on it where you can see.
She’s not that into you. She also seems childish. It would be better for you if you moved on. Patience is good in relationships when someone is trying, but she’s already checked out and treats you with complete disrespect. There is nothing here to save.
i think that you should maybe wait to confront her because the picture of just him and her doesn’t sound that threatening, but you do have a reason to be a bit annoyed about it as its not very normal. personally i dont think shes worth your time if shes threatening to leave you just because youre not vegan, she should respect you rather than judge you