The pop star/actress was one of several stars who hit up Bey's show in Paris Friday -- just one of many stops on her Renaissance tour -- and while she was crossing the main stage area ... you can see she had a heated confrontation with one of the dudes trailing her.

He's with security, no doubt, and it seems he might've done something to piss Selena off -- 'cause she was definitely chewing him out. You can see he comes over closer to talk to her and then points in another direction. As for what this was about ... there are some theories.

Some on social media are claiming the bodyguard -- who was seen following Selena at other points in the show, leading sleuths to believe he might actually work for her personally -- aggressively shut down a fan who was trying to reach/talk to Selena as she was passing by.

There's no evidence of that from what we can see, but it's plausible -- she was getting pretty close to the front row crowd as she was heading to her VIP area ... and was even high-fiving several along the way. Maybe somebody got a little too handsy past the barricade.

There's one video circulating that purports to show a group of people attempting to mob her on the stadium floor ... but, to be honest, it's hard to make out what's what in the clip.

Of course, right on cue, there are some haters who are criticizing SG over this -- suggesting she overreacted to a tactic used in her best interest -- and others who are praising her. But, again, nobody seems to actually know what this was all about -- it's pure speculation.

Now, if there's any truth to the security/fan story ... you'd naturally compare it to what Taylor recently did at one of her shows -- where she stuck up for a concertgoer who had a run-in with security down on the floor. That incident was much easier to parse, though.