Rob Kardashian & Blac Chyna's Breakup Leads to Fight with Sisters

Rob Kardashian and Blac Chyna are seriously on the outs, and Kylie Jenner got caught in the cross fire when she planned a baby shower for Rob sans Chyna ... TMZ has learned.

Sources close to the Kardashians tell us Rob got pissed at his sisters Monday -- especially Kylie -- for what he considered a slap in the face ... the Chyna-free shower they were throwing. We're told the sisters only planned separate his and hers showers because Rob and Chyna haven't been speaking for months.

They briefly reunited for a dinner the night before their reality show premiered, but other than that ... they've been split.

Still, Rob was pissed when he learned BC wouldn't be at the shower -- so he skipped it, and dialed up some revenge by tweeting Kylie's phone number. Thing is ... we're told Chyna had agreed separate showers were best.

Rob wasn't told that until after the fact, and we're told he's still pissed. We're guessing Kylie will have to get that number changed.