AITA for not letting my girlfriend have a slice of my birthday cake after what she did last year?

Last year on my birthday my girlfriend (26F) ate the last piece of my cake while I was at work. Not just any cake, my favorite cake that my mom makes from scratch every year. I had specifically told her I was saving that last slice for when I got home.

When I asked her about it she just shrugged and went “Oh I didn’t think you’d care.” No apology nothing. I was pretty annoyed but didn’t make a big deal out of it.

Fast forward to this year my mom made me the same cake again. My girlfriend asked for a slice and I told her “Nah after what happened last year this one’s all mine.” She laughed at first but when she realized I was serious she got pissed. Said I was being ridiculous and “holding a grudge over cake.” I told her it wasn’t about the cake it was about her completely disregarding me last time.

Now she’s barely speaking to me and a couple of our friends are saying I’m being petty. I don’t think I am though. If she couldn’t respect it last year why should she get any this year?

AITA?