Just ask them if you should bring your own food (because you don't eat ? Whatever). next time she invites you. Maybe they will get the idea that it is sort of unexpected for a regular guest

I don't think it's rude. Perhaps she has a little anxiety about it and wants to offer you an "out" in case you're sick of catering to her needs (which it does sounds like so perhaps she has picked up on it). You could always get whatever meat pizza everyone wants and also order one cheese, put a few slices aside for her, so the cheese isn't gone before she gets a chance to have a couple slices.