You're compassionate to appreciate that this conversation may be hard on your mom. She may feel uncomfortable re-visiting this painful time in her life. She may feel guilty for choosing this man to be your father. She may feel sad about the mistakes she's made and how they impact the both of you today. She may feel defensive if you question the decisions she's made. She may feel that you don't appreciate all that she's done.
Give your mom a heads-up that you would like to discuss your dad with her. Instead of ambushing her with difficult questions, give her some time to collect her thoughts and put things in context. Give her time to formulate the overriding message she wants to impart to you, not just the details of the situation. After all, what she's learned from her journey in far more significant than whether or not your dad had a girlfriend.
Use this opportunity to understand your mother in a new and deeper way. Don't waste it by simply making it a fact-finding mission about your dad. Make it about getting closer to your mom. After all, she's the one who's been there for you.
Sometimes fatherless daughters fall into the trap of becoming preoccupied with their absent dads. Because they're out of grasp, they seem more mysterious and desirable. By doing this, though, a fatherless daughter can make her mom feel devalued.