ROB KARDASHIAN has accused his girlfriend . . . British singer RITA ORA . . . of some serious cheating. Yesterday he Tweeted, quote, "She cheated on me with nearly 20 dudes while we were together . . . But I mean 20?!!!! How can a woman who is so busy trying to start her own career have time to be with so many dudes while in a relationship?! I am actually disgusted."