Ariana Grande and Big Sean have "gotten serious very fast," an insider reveals exclusively in the new issue of Us Weekly Credit: Chelsea Lauren/WireImage

These two only have one "problem" — they can't get enough of each other! Ariana Grande and Big Sean's new relationship is in overdrive, and it's because they were friends first. An insider recently revealed to Us Weekly that "it's gone from 0 to 60 fast" between the two, and they had "chemistry immediately."

Grande, 21, and the 26-year-old rapper first collaborated on her single "Right There" in 2013, but they only recently got romantic after Grande split from BF Jai Brooks last month and Big Sean (memorably!) called off his engagement to Glee's Naya Rivera in April.

"Sean asked Ariana out in a romantic way and they went to the movies," the confidant explains to Us. "Now it's gotten serious very fast. They are much happier than in their previous relationships."

Grande and Big Sean have kept coy about their budding romance, but were recently spotted holding hands backstage at the MTV Video Music Awards on Aug. 24. Days later, the "Bang Bang" singer said she's "happy" when asked about her love life on the Today show.