I'm upset at my mum for asking me to buy her a gift on my birthday.
What about this is upsetting you the most?
I don't want to buy a gift.
I understand, it's your birthday, so it's you who should be getting all the gifts! How about try explaining that to your mom, and you can always gift her something at another time.
I'm fine. She was always bad with gifts and it was always a disappointment. And what do you know. She's my Mum not my girlfriend.
So are you okay with buying her a gift instead? That's pretty sweet of you to be selfless.