Justin Bieber -- Flips Over Pics at Stable ... Y'All Better Gitalong, Bitch! [VIDEO]

Justin Bieber acted like a horse's ass ... calling a woman a "bitch" for trying to take his picture Monday at a riding stable -- just hours before his alleged confrontation with a mom at a mini-golf course.

Melinda Giel-Murray tells TMZ, she and her horse were at the Circle K Stables in Burbank, CA when Bieber pulled up to go horseback riding.

Giel-Murray says she whipped out her camera, but Bieber immediately freaked -- and in the video you hear him say, "that bitch over there" ... as he points out Melinda to his posse.

Giel-Murray claims 4 security goons surrounded her and tried to intimidate her, but she stood her ground ... telling them there's no law against taking pictures.

She feels Bieber was acting "very strange and nervous." His crew eventually drove to a different part of the stable to mount up ... shirtless.

A few hours later, Bieber allegedly confronted another woman trying to snap a pic at a nearby putt-putt course. Police are investigating, but a park employee tells us Biebs didn't touch that lady.