Rob Kardashian got tipped off that Blac Chyna was allegedly cheating on him by the baby mama of "the other guy."
Sources connected with Rob and Chyna tell TMZ ... Rob got a DM 2 weeks ago from the woman who had a baby with rapper Ferrari. She claimed Ferrari and Chyna had been intimate for 2 months.
Ferrari's baby mama blew the whistle just after Rob reconciled with Chyna. He wasn't sure if the woman was telling the truth, so he began confronting people, starting with Ferrari.
We're told Ferrari denied he was hooking up with Chyna, but Rob persisted and then confronted her. We're told Chyna also denied it.
Fast-forward to the weekend, when Ferrari posted a pic of himself in Chyna's bed ... especially galling because Rob says he paid for the bed. Ferrari was wearing Rob's Versace robe ... which enraged him.
Rob claims he went through Chyna's phone Tuesday, and found texts, nude pics and FaceTime photos between Chyna and Ferrari, and he declared war.