This wasn't my girlfriend but it was a spectacular sight anyway.
This was in the mid 90s and I was at an outdoor pool with a colleague and her huhsband who had their friend and his girlfriend staying. This couple weren't the most exciting pair and she wasn't a great looker to be honest. It was the end of the day and we and a couple of kids were the only people there. I joined them as they were about to leave and I remember it very clearly. She climbed the ladder at the deep end and I noticed she had one of those very high cut speedo type one-piece suits that were popular at that time for proper swimming. There was a
lot of thigh on display and she walked towards me and to the changing room. She was a "big hipped" girl and I was impressed that this very quiet, very conservative girl would wear a suit like this and as I watched her walking away she had a fantastic wedgie happening. Her bottom cheeks were almost totally exposed and wobbled beautifully as she strolled back. I still wonder if it was her own swimsuit or one she'd borrowed while staying with my colleague. Either way, it really didn't fit
. One of the boys there looked embarrassed, but she didn't make any attempt to pull the material back or hurry to cover up.
Although she wasn't anything special, I found this an incredibly erotic experience that still turns me on. I think it's that contrast between a quiet, shy girl being very revealing and not appearing to be either aware or bothered.