The actress detailed an upsetting story on social media Saturday, describing how a girl kept pressing her for a pic ... but apparently wouldn't take no for an answer, and hit record instead when she got denied.

To that KP writes, "No means no, even when it doesn’t pertain to sex. I was at the bar the other day and this girl asked me three times for a picture and I told her three times nicely that I did not want take one with her. She still preceded to film me against my will."

She adds, "If I went off on her I would’ve been wrong, so I just nervously laughed while my privacy was invaded upon." Unfortunately for celebs, this is true -- if you're out in public, you're fair game for anyone with a camera ... even if it feels invasive, like this clearly did.

Keke further addressed it on IG ... "Clearly I’m still upset about it cause I hated that I smiled, but that is my defense mechanism to laugh or joke in an uncomfortable situation and it misleads people every time. Literally I could be wanting to scream, and on the outside I’m still performing. The fact that I tell people no at all is therapeutic progress for me lbvs."

She goes on to explain boundaries are hard for people who defer to people-pleasing or who don't want to steer into conflict -- which she says isn't a good practice, but a real struggle.

Here's the fascinating thing about this ... Keke has very publicly dealt with this exact sort of issue before, albeit in a much more insidious setting -- at least based on what she claimed.

You'll recall ... she accused Trey Songz of "sexual intimidation" after he allegedly coerced her to be in a music video of his against her will -- to the point she claims she had to hide in a closet from him ... something Trey later downplayed and seemingly denied on social media.