How would you act if you found out that your boyfriend was cheating on you with a girl younger than him when i?

Okay so TWO days after Christmas Devin (my bf) calls me and tells me that we should just be friends he told me it had nothing to do with this girl younger than him (lindsay) so i hung up on him and the very next day; my friend told me that this girl she knows saw Lindsay and Devin flirting and huggin on each other. Thats why im so pissed! btw- Lindsays brother is one of my best friends! And i really like his older brother Loy! He is sooooo hot!