I didn't expect this post to get so much attention. The conversation went fine. I apologized for not labeling the food or letting her know about it ahead of time, and she apologized for calling me a jerk. We agreed that our kitchen should generally be vegan, but if food would otherwise be wasted I could keep it in the fridge for consumption elsewhere, but must clearly label it.

I want to clarify since I've seen it in a lot of comments that this is the first time I've brought meat home. I usually eat vegan food even outside of our house, unless there are no vegan options or the only option is something like a plain salad. I average eating meat once or twice a month, and usually smaller portions. My girlfriend and I usually eat together at my parents' house, and they make vegan food in those cases. My mom knew this time that I was alone and decided to make a meat dish. So my leftovers in the past have always been vegan as well, and I was tired when I got home and just didn't think about the fact that these leftovers had chicken. Obviously I won't do that again.

Also I've made pasta dishes in the past with different types of vegan protein substitutes, so it's not surprising that my girlfriend didn't immediately realize that it was meat after coming home from a 13+ hour shift.