Williams went in on Chyna and Rob for a solid 5 minutes on her show Friday -- saying Chyna's only using Rob for personal gain -- and calling the "old" Rob thinner, smarter and more confident. Chyna clearly got word of the show segment, because she went ballistic on Williams Saturday in an Instagram post.

Chyna fired off, "so funny how you love to talk about Rob and his insecurities like at a point in time you wasn't a fat 400 pound ass bitch on the radio!" She continues, "You go out there talking about my family like you just hit the pipe backstage!"

Chyna's most creative zinger lands at the end ... "your horse face ass woke up on the wrong side of the stable and decide to go overboard with your f*** boy comments. Bitch f*** you!"

Yikes.