When someone's butt is suddenly visible with the type of pants they are wearing, unlike any other day. Cake is referring to one's butt.
Friend 1 (Girl)- Do these pants look good on me?

Friend 2 (Girl)- Dang girl you lookin like a snack!! (Lookin hot

Friend 3 (Girl)- YeeeezzzzZzuh!! Is it my birthday?!??! 'Cuz I see cake!!

If someone says this to you it means that your butt or booty looks good in what ever pants you're wearing. They're complimenting you unless said differently lol.
by bae with boobs bruh1738 December 31, 2018
Get the Is it my birthday?!??! 'Cuz I see cake!! mug.