Tyga & Kylie's Spies Stole Blac Chyna's Party Theme for Her Son

Blac Chyna is pissed at Tyga for jacking her Ferrari birthday party plans for their son, and adding his own bells and whistles to outdo her ... and she smells a rat behind the whole mess.

A source close to Chyna tells TMZ ... she believes a production staffer on her reality show fed Tyga and Kylie Jenner all the details of her party for King -- which she spent 2 months planning. We're told Tyga and Kylie were invited to the Saturday event, but no-showed ... then threw their bigger copycat bash the next day.

Chyna says she's happy her son enjoyed the birthday double up, but as a mother ... it was a slap in the face. Plus, it's insult to injury if someone from her own show tipped off her enemy.

Co-parenting. It ain't for everyone.