Big Sean tweets his displeasure after Justin Bieber grabs GF Ariana Grande on stage!

Justin Bieber joined Ariana Grande for a surprise appearance during her performance at the Forum in Los Angeles yesterday. During a duet with Ariana, Justin grabbed her from the back, grinding into her. Ariana's boyfriend – Big Sean who watched from the audience wasn't pleased with it and took to twitter to let everyone know how he felt about it.