>>33209797 (OP)Before you get married make damn sure you have a prenuptial agreement in case shit goes south. This is definitely a big red flag.
Right now focus on calming down. Maybe go out, clear your head. Later on, when your calm, cool, and collected, talk to her about exactly what happened. Be calm and polite but honest about it. Find out exactly what went down between your fiance and her ex.
If she won't be straight and honest with you, try asking the ex himself, in a polite and respectful way (he's more likely to be honest if you don't come off as aggressive) if things don't feel right, consider breaking things off.
If you decide to break things off, get a lawyer. Make sure she gets a paternity test when she gives birth to make sure it's yours, and if it is your son, you can still be a father to him/her without being married to someone who you don't trust.
Ultimately, the only one who can/should decide if you should stay or go is you. Think long and carefully about it, and do what you think is right.
I'm sorry you're going through this.