AITA - girlfriend asked if I could fake a sick day for her birthday, I told her that I couldn't?

I (m31) am a school teacher and she (f33) works in tech though works from home 4 days a week. So her birthday was midweek and she asked me if I could take a sick day so we could do something on her actual bday. I told her that I would love to spend the day with her but in my current job even taking genuine sick days are a real pain. I have to create the cover work and resources for each lesson, writae detailed instructions and print everything off for the cover teacher. Once I return from illness I invariably have to teach the same lessons again as the cover teachers often fail to motivate the students to do any work. Having a sick day is more hassle than it is worth and I only take them if I am very ill. It is not a case of simply texting or calling in, saying I am ill and thats that like my girlfriend does in her job.

I informed her that I was going to cook her food on the evening of her bday and that we would open her presents (I also bought her a cake as a surprise). We had previously planned to go out for a day on the following weekend. When I told her that I wouldn't be able to take a sick day I thought she would understand and accept that we would go out on the weekend instead.

Boy was I wrong. She berated me; telling me that I didn't care about her and that it was ridiculous that my job would not allow me a day off (we don't have the option to have random days off, we have predetermined school holidays and weekends). This shook me because I understood her proposing the idea but I never imagined that it was an order rather than a request. She also asked me if I had bought her a cake to which I stated I hadn't (as it was a surprise) and she again accused me of not caring and being selfish.

I understand that doing something on your actual bday is preferential to doing it later but she is a 33 year old woman and surely understands that this is not always possible. I oersonally feel. she was. immature but what do you think? Should I have taken the sick day?

AITA - girlfriend asked if I could fake a sick day for her birthday, I told her that I couldn't?
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