What a day. I knew the transition back to my house would be difficult after 4 days with dad. Nevertheless, today really tried my patience. I stood in the rain not showing any signs of annoyance when Sabrina wouldn't take her bookbag off her back in the car after school. I did almost as well on the way TO school when she kicked me in the chest trying to get her out of the car.
When we got home from school she was really cute (after she ran away in the parking lot and refused to hold my hand) and interested in engaging with me for a while; we explored the nooks and crannies of my apartment building, but at some point she ran off, then stuck her tongue out at me, ran off more, stuck her tongue out again, and then just refused to listen to any requests I made.
We then talked to mom tonight after dinner, and now everything is clear. A little more back story: Sabrina wanted to watch a movie when we finally got into the apartment and I wouldn't let her. So when Sabrina was talking to mom, she complained that I didn't let her watch a movie. Mom was upset about this. To paraphrase mom: "We just give her anything she wants because crying is bad for her asthma and she's just a really special kid. It won't work to not reward negative behavior because it will just make things worse."
In other words, Sabrina has been spoiled for the last 4 years. Fun.