My(25M) girlfriend(25F) called me fat. Please guide?

My girlfriend (25F) and I (25M) have been in a relationship for about 1 and a half years.

This morning she said that I need to suck my tummy in more often because my gut hangs out and it’s ‘disgusting’ . She also adds that she’s not being mean this is just how she honestly feels and she can’t change that… This hit me quite hard.

This isn’t the first time she has mentioned me being fat, she has previously said maybe I wouldn’t have such a beer belly if I exercised more. Which I explained I did not think this was an appropriate thing to say in a relationship (or true) and we should be supporting each other.

The problem is I don’t think I’m fat, I’m 177cm tall and weigh about 69kg and I exercise a fair bit (cycle to work 30mins each day and run ~40km a week) and don’t drink, so don’t have a beer belly. While I don’t think I’m skinny (or muscular) I don’t think I’m by any stretch fat.

This is my first serious relationship so I’m not sure what to expect, and would like advice.

Are situations like this just part of a serious relationship?

what can I do? I’ve previously told her I don’t think comments like this are appropriate and true. And she still feels this way… so I’m not sure what to do.