Soooo we all know Justin and Selena broke up. No big deal. But we don't know exactly why! Until now!

Star Magazine is reporting that Justin and rapper Lil Twist met nursing student Milyn “Mimi” Jenson around 6 p.m at the Beverly Hills Four Seasons.

“They all went to buy weed at a smoke shop on Olympic Blvd. in Hollywood,” the insider said, “then they got food at McDonald’s and went back to the hotel.”

At the hotel, the group smoked marijuana and drank a chemical concoction known as “sizzurp” throughout the night. From there, they went to the Baby singer’s $6.6 million Calabasas mansion, where Justin opened up about his fondness for tattoos and Xanax, as well as his plans to build a hookah room in the house.

After that, Justin held Mimi’s hand, and showed her the breathtaking view from his backyard. The insider said that Mimi said Justin was really kind, romantic — and stoned — after he’d been “smoking pot all night.”

SOOOOOO BOOM!!! THAT apparently is why they broke up. Selena found out about this "nurse" and ended the relations**t!