My mom took a bite of my birthday cake that i bought myself for me

Yesterday was my birthday and to celebrate i bought 2 slices of cakes after work for me to enjoy, but because it was late i put it in the fridge to eat later. I was a little stressed recently so when I got off work today i feel happy because im about to enjoy some cake which i havent had in a while, but when i open the fridge lo and behold my mom took a bite out of them. Yes she knew it was my birthday yesterday, no i dont think she cared. I might be a little overreacting but im feeling more than a little upset over it. I know its not a big deal but its just the principle of the thing, you know ? My mood was immediately shattered

Just to add the day of my birthday she asked me to take HER out for dinner, i think shes mixing up whose birthday it was