Sources close to Rob and Chyna tell TMZ ... the couple had a heated argument about the sexually suggestive lyrics in Game's diss track, "92 Bars" ... because Rob had no idea Game took her down.
We're told Rob was weirded out because Game had spent so much time with his family, and he felt it's something Chyna should have disclosed early on in their relationship.
She begged to differ, claiming it went down long before she hooked up with Rob.
The lyrics go, "Blac Chyna head the bomb, Al-Qaeda."
After they argued, they agreed the common enemy was Game, who never gave Chyna a heads-up.
Chyna's 8 months pregnant with their daughter.