My deepest, darkest secret

Hello, I have 1 kid a baby boy, and my girlfriend wants more, but I already find it hard enough with 1. He’s 2 in 2 weeks, I love him with all my heart and would do anything for the boy, but when I realised a yearsh add on a few weeks that I didn’t want any more kids, I got my tubes tied without telling her, my friends or my parents…. A year in my kids coming up 2 News comes along I’d never expect My girlfriend is pregnant… it’s apparently mine but come on… and within a week of finding out… she tells her parents, RIGHT IN FRONT OF ME…. IM SPEECHLESS .I know it’s not my kid…. I’ve not told her yet… what do I do? I didn’t want to just confront her, I want to do it in a way that amuses me but I can’t really think of one except from finding the guy she’s sleeping with and smacking his jaw until it’s crumbled cookies (infront of her) MIND THIS GUYS BEEN ROUND MY KID WHILE IVE BEEN WORKING… so don’t say that’s bang out of order YOU ARE