Like she was pregnant this whole time and you weren't aware that she was pregnant, now the kid is born and she saying its yours? Time to go see Maury.
I guess initially I'd be pretty damn pissed. I wouldn't believe her and I'd call her a slut. Keeping that shit from me, but then I'd be like "Holy shit, I made that kid." and then I'd take care of it. I don't know what kind of relationship you have with the mom, like if you're cool with her or if you absolutely hate her so that might change shit. Some more info would've been helpful.
If I knew there was a possibility that it wasn't mine I'd have take that paternity test shit, cuz i ain't taking care of some random kid that ain't mine. So, how did you actually react?
Congratulations on being a dad. Cigars for everyone.