You want to play the a bit ashamed about his past jerkboy. You don't feel good about cheating so you don't want to give too many details BUT you don't feel too bad about it because a)she was so hot b) you are so awesome for having various girls that you get into c)whatever other reason. So you basically smile and ignore it somehow, either by changing conversation, accusing her of already knowing because she's stalking you (only if you have imposed this frame before), I don't know man, be creative about it, but never be too direct.
Many, many times, when I have find myself in a similar situation, all I do is put a ****-eating grin
and change the topic. I learnt this from Harrison Ford and it works like a charm.
2 quick examples:
girl a) first after-bang cuddling time in my room, she finds a hand-made scarf made by...MY MOM! She asks, and I decide to play the "I'm getting laid sooo much" role, so I say "oh, a male friend made it for me", she obviously does not believe me and replies "male? that would be the first time I hear something like that". She looks at me expectantly, I put on my Trump/Han Solo face. Back to sex.
girl b) I'm driving her back to her place on the morning, this came from social circle gaming, I had like a ****-fvcking-ton of value on her eyes and she knew I had a "gf". Anyhow, she can't hold her hamster anymore and asks it "weren't you with another girl?". I proceed to smile smugly whilst talking about how fun mercedes are to drive. She kept on texting me to meet again for weeks, I refused every time, too risky.
Now you know what to do the next time a dangerous topic appears in the horizon. Smile, change topic.