Background: 13 year relationship with my fiance, some points off and on, but mostly on. Live together in a house we bought, have a dog, etc. I don't like beer, but always like to taste everything for science.

Lead up: Go out to dinner at a pub. Reviews said service can be slow. I order water, she orders random draft beer. I get my water right away, it takes her 15+ minutes to get the beer. We are sitting next to the bar and see those drafts going out left and right, seems like just busy. Joke around, speculate why it's taking so long, having a nice time. During this wait, we are sharing my water and since she drank it last, glass was on her side of the table. Waitress puts beer in front of me as from her perspective it made sense since I didn't have a drink in front of me. 

Incident: She walks off, we look at each other. A few seconds go by, fiance doesn't reach for her beer, I decide to do it for her and decided that while I have the beer in my hand(all about efficiency), might as well take a small(really small) sip to taste since I would do it at some point anyways. Put the glass on her side of the table and see her giving me a "look". Demeanor immediately changes and I start getting the cold shoulder. A bit confused, try to pry out what's wrong, but just silence. After a few minutes she asks me do I really not realize how rude I just was. I am still not 100% sure what I was rude about, but obviously something to do with me tasting her beer.

Conclusion:  Giving me cold shoulder for rest of the night, maybe 10 words exchanged max. Next day similar, again brings up that I was rude. I disagree. Me best guess is it's not that I had a sip as we share food all the time (see water above), but maybe it's because I had the first sip especially since it took so long to get to the table. I don't believe this exact argument has come up before in the 13 years, but she has expressed annoance if I try to eat off her plate at the same time as she has a fork in her own plate (so fine sharing as long as I am not disrupting her creating a "perfect bite") AITA for taking the first sip or overreaction on her part? 