An Internet radio host in Charlotte, North Carolina claims that JUSTIN BIEBER was rude and insulted her when she tried to work out at a local gym.

--Colette Harrington wrote an angry post on her website Tuesday, saying Justin was working out with a trainer and one of his backup dancers, when she approached and introduced herself as a radio host.

--Justin replied, quote, "Good for you."  Then she asked if he would take a picture with her, and he said no.  Colette says she was cool with that, and just went ahead with her workout.

--Colette says she counts her reps out loud . . . and Justin and one of the other guys started mocking her by counting out loud with her, and then shouting "Shut the [eff] up" every time she hit 20.

--She said, quote, "I [was] ready to punch one of them in the face or cry.  I could not decide which since I felt like I was back in grade school and being made fun of."

--She also said the two guys were talking about spitting in people's drinks, and at one point, Justin guzzled a bottle of Gatorade and then spit in it.

--Colette says she left the room for a while, then came back in when they left.  And to add insult to injury, she accidentally took a drink out of the bottle Justin had spit in.

--In her Facebook post she threatened to sell the bottle on eBay . . . but she later claimed she wasn't serious about that.  (--You can read her post here.)

--Justin's rep admits that Justin and his trainer were at the gym . . . and that they were making snide comments.  But he says they were ripping on EACH OTHER, and NOT on Colette.