Lately my girlfriend has been getting angry at me for petty things, like, not replying fast enough to messages, and then refusing to talk to me for hours over such meaningless squabbles. Anyway, she’s also been hanging out with this guy who I’m pretty sure fancies her quite a lot lately, in fact till 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 its 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 
In my opinion, yes, this is a valid reason to be annoyed and concerned. Your feelings/instincts are telling you something just doesn’t seem right with what is going on. You’ve got to communicate though, let her know how you feel and give her a chance to explain her side. (Hopefully without causing a fight or getting the silent treatment.) You deserve to be happy, comfortable and secure in your relationship. I hope it turns out well for you.