Mum Cucked Me

This was humiliating, so I was about to enter university and we were having abit of a family and friend leaving party last June.
Main guests were my girlfriend and some other friends at my mums house
Bit of context first my girlfriend has a thin healthy figure with long black hair and my mum has a voluptuous figure with long brown hair and still looks great. They've always had a great relationship doing stuff together.
Tonight everyone was dressed up, me and the two other guys were in nice shirts and my girlfriend was in a blouse with nice jeans as was my mum.
Me and the guys were drinking and talking then I noticed my girlfriend had gone off somewhere. I excused myself to the toilet and I came to a weird sight. My girlfriend was sat on my mums bed rubbing my mums big soles on her lap, I couldn't quite hear what was being said but she was clearly enjoying it as much as my mum was.
I took abit of time in the bathroom as I needed a you know what. Noise just attacked me on the way out, I peeked through and saw what they were doing now,
My girlfriend was now naked as was my mum and she was riding my mum! I'd never seen my girlfriend moan and make sounds like that and my mum was no different, they even switched positions with my mums huge ass now above my girlfriends small one.
I left and tried to finish the night as normally as possible which I managed, my mum and girlfriend came down later and acted as if nothing happened. That was last June and I've been to university as planned but am now home so thought I'd let this off my chest after seeing them both again. It was so unexpected and embarrassing