So this happened a few days ago and I'm trying to grapple with left over feelings. 

Background: My partner and I had a few weeks break in the new year and had planned a semi-local two week trip. At the same time we planned for my parents to come for one of the weeks and are visiting from my home country. We planned to stay together and do activities in that time. My Mom generously said my Dad and her would cover things while we were together. Awesome. I'm super thankful for them doing this. So one of the days we finish a small fishing trip and some wine tasting and my partner and I had discussed maybe doing dinner just the two of us. I ask/let my Mom know that we wanted to have a dinner out, just the two of us and they are all sweet with it and say they're planning on eating in. Side note, my Dad still cooks like 6 people still live in a house so already on our holiday we have too much food. So we eventually find a place, order, eat and have some leftovers of a half n' half pizza. We end up going back to the AirBnb and I go to chill in the our room. My Mom comes in chewing on a piece of my partner's pizza and sits down on the bed staring at me. I kinda gave her this look to which she says something along the lines of "What..why are you looking at me like that/what's the matter??" I think I said something like "Well, that's Dan's pizza... you should have asked." Immediately she gets angry/hurt/defensive, says something (can't remember), I respond then she leaves. Suddenly outside the room on the balcony I hear her talking quite upset to my partner about what I said.   


Dan didn't know at all what was going on. He had come out of the toilet and was having a smoke when she came up. (D for Dan, M for Mom)

M: "Sorry Dan I ate a piece of pizza, I hope thats fine"

D: "Absolutely, not a problem, but I would liked to be asked in the future."

M: "Why?"

D: "Its just common courtesy."

M: "But we've paid for everything. We've paid for the trip, accomodation, activities. I don't have to ask for a bit of pizza."

D: "Its still common courtesy to ask regardless of what has been done."

M: "Well I've given you all this and I expect the same in return."

At this point we were just trying to get across that common courtesy should be used no matter the case. We paid for the pizza and still may have wanted it, but were happy to share. We tried giving other examples but it boiled down to her believing/expectation that if she gives then she expects to receive no questions in return. We tried to explain that she made it seem like listing what they've paid for was more a gift with strings attached rather than an unconditional gift. We also said that while those might be her expectations, other people (like us) don't necessarily expect the same of others.

It ended with silence, in the morning it was the same and is still technically unresolved but I was left feeling like I wasn't wrong but guilty because she/my Dad had paid for everything. Reddit, am AITA somewhere in this?

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TLDR; Mom and Dad paid for part of a two week trip we (including my partner) spent together. Mom got upset when I said she should have asked before taking a slice of pizza from our dinner out. My partner and I were just bringing up common courtesy while she felt that paying for accommodation, activities and groceries entitled her to a simple slice of pizza/not needing to ask. AITA for saying she should have just asked first.