Heather McDonald -- So Over Chelsea Handler ... and Headed to TLC

Comedian Heather McDonald is teaming up with Margaret Cho for a new show with TLC ... this after our sources say she was completely blindsided by Chelsea Handler's move to Netflix.

According to our sources, McDonald -- a longtime fixture on "Chelsea Lately" -- and Cho are doing a show called "All About Sex" that will air Saturday nights. We're told their show will bust on the show airing before it ... "Sex Sent Me to the ER."

Our sources say McDonald was relieved to land the gig after she was given ZERO heads up from Handler that show was ending.

TLC has yet to announce the new show, but we're told it will premiere in January.