Justin Bieber Hooking up With Rihanna?

Life & Style is reporting that Selena Gomez broke up with Justin Bieber after two years of dating because he had cheated on her in the early stages of their relationship -- with Rihanna.

"It has devastated her," a friend of Selena and Justin's reveals in the new issue of Life & Style. "It really cut like a knife because it happened in February of 2011, when Selena and Justin had been dating for months and were falling in love. It's caused Selena to question their entire relationship."

After flirting at a basketball game in LA on Feb. 20, 2011, the friend tells Life & Style that he and Rihanna hooked up.

"Selena thinks maybe they've been hooking up all along," their friend tells the mag.

"During breaks, they'd sneak away to the Hotel Giraffe," the source tells Life & Style. "They were alone -- their security made sure of it. Hotel staff blocked off the entire floor for them."

A rep for Justin denies anything physical happened with Rihanna