Scott Disick -- Kourtney 'Fuming' and 'Worried' He's Hooking Up and Off the Wagon

is telling friends she has no doubt ... Scott Disick is hooking up with the chick he was all handsy with in Monte Carlo, but she's also deeply worried he's lapsed and fallen back into substance abuse.

Our sources say Kourtney believes the woman, celebrity stylist Chloe Bartoli, is an opportunist and taking advantage of Scott, whom Kourtney believes is getting wasted again. We're told she knows Chloe and, as we reported, Scott knows her as well ... they even dated.

Our sources say Kourtney's sisters have the same view as her and on Friday they all went from simmer to boil, targeting their anger at both Scott and Chloe.