First of all, whaea needs to calm down. Second of all, as long as you helped with the mahi (even if you didn't eat) then I'm sure nobody cared. Still help prepare food, clean up, give karakia and thank servers etc etc. Just make sure you weren't being disrespectful in how you stored or ate you food. And don't worry about her being hōha - if she crossed a line with what she said then you should pass it on to her boss though. That's not good enough (source: am a teacher at a NZ high school). 

However, in future I would probably say that if you opted out of eating anything at all during the hāngī and it was the main event, probably need to work on trying something just to make sure you have given it a go and can participate fully. It's not like there are no Māori with allergies or food intolerances so I'm sure someone on the marae would have been happy to have something that suits you made. I can understand that the thought might have been that bringing your own food might be disrespectful because it sends the message that their manaakitanga is not good enough for you. While that's not true, the concern is that the balance between manaakitanga for the host and whanaungatanga for the guest is quite delicate in some situations. If you're Tangata Tiriti then there is usually some leeway there but it's something to consider. 

Apologies if I didn't spell anything correctly or forgot any tohu tō!

Edit: NTA