ZAYN MALIK from ONE DIRECTION cheated on his girlfriend recently . . . and after it was over, he was a total JERK to his side-action.  Unfortunately for him, she took PICTURES of him in bed, then posted them on the web.

--A waitress named Courtney says she went back to Zayn's home after a party . . . and after he assured her he was single.  Not being a One Direction fan, Courtney says she didn't know any better.

--Even though Zayn is 20 years old, his house was like a kid's playroom.  Courtney says, quote, "He had remote control cars and skateboards floating around the house to play with.

--"There was a 'Star Wars' mask . . . and there was a life-size Kung Fu Panda under his stairs.  It was a fun house."

--Then came the sex, which Courtney says was AMAZING.  But apparently Zayn's not much for afterglow.  She says, quote, "Zayn made me feel special . . . but after sex he just said, 'Put your clothes on, I'll get you a taxi.'"

--Before that happened, though, Courtney took a couple pictures of Zayn asleep in bed, post-hook up.  (--You can see them here.)

--Zayn's girlfriend is PERRIE EDWARDS, the blonde from the British girl group LITTLE MIX.  They were put together on the British version of "X Factor", just like One Direction was a season earlier.