AITA for staying quiet about my girlfriend's friend coming on to me?

We went to stay with her - her partner (i think they're married) was away so it was just her. I arrived early afternoon because I managed to get the half day off. My girlfriend arrived 9pm.

She has always been a bit flirty and I guessed she liked me but then again I've thought that before and been totally wrong. She was really smily when I got there, asked if I wanted anything, food, water, a beer etc... "anything". I said I'd take a beer and she got me one and repeated "anything" again. She then asked when _____ was arriving and I said it would be 9-10 pm. Then out of nowhere she asked me if I wanted my d*** s**cked. I sat there stunned and then laughed awkwardly and she said "yeah if you want a bl****b I can give you one now. ___ doesn't have to know. I'm so jealous of her". I turned her down and she said if I changed my mind I could ask any time.

My girlfriend arrived 9pm and we had a nice weekend and she acted like nothing happened but I have no idea if I should leave it and forget it ever happened.