Justin Bieber goes off on woman at a gym?

Justin Bieber went off on Colette Harrington -- a NC radio host -- at a gym in North Carolina Tuesday. TMZ reports on the woman's claims. Harrington says Bieber ... mocked her workout, cussed at her ... and possibly even spit in her water bottle

According to Colette, Bieber arrived at the gym at the Ritz-Carlton Hotel with two other people -- one appeared to be his trainer and the other appeared to be a backup dancer or singer.

Colette says she approached Bieber and asked for a picture, but JB said no.

She says she continued working out, but then heard Justin and his friends mocking her workout routine.

Colette explains, "I always count out loud my reps. Then Justin starts imitating me and whenever I'd get to 20 he and his buddy would say 'Shut the F'ck up.'"

Colette says she ignored the group -- but claims they continued to mock her as she counted out her reps.

Then, according to Colette, someone ... and she thinks it was Bieber or someone in his group ... SPIT blue Gatorade into her water bottle while she wasn't looking ... and she actually DRANK the spit-orade.

"I did not look at my water bottle before taking a drink."

Colette says she filed a complaint with the hotel after the alleged incident.