Oprah Winfrey was MIA from CBS this week, despite previously being announced as a guest to plug a new book -- and the reason is simple ... she was pooping and barfing.

O's bestie Gayle King got a little TMI during "CBS Mornings" Tuesday -- where she and the other anchors were hosting author David Wroblewski ... who's got a new book out that Oprah trotted out as the latest addition to her famed book club.

Gayle explained Oprah's absence -- saying she'd caught a bad stomach bug that landed her in the hospital. Then, a bit more detail about how it was affecting her ... and it's gnarly.

She explained ... "She had some kind of stomach thing -- stomach flu -- stuff was coming out of both ends. I won't get too graphic. Needless to say, she ended up in the hospital, dehydration, had to get an IV, it was a very serious thing."

Oprah's on the mend, according to Gayle -- who hoped her BFF wasn't rattled she put her health news on blast. GK also said she hoped O wouldn't mind this info was out there.

Thankfully, David was unfazed by Oprah's absence, as he happily promoted his new work, "Familiaris," on the morning program.

It also probably helps that O had already talked about his book on social media, with a pretty solid caption that highlighted how good of a read it is.

As far as Gayle dishing the dirty deets about Oprah's crap ... hey, they're like sisters -- and all our siblings embarrass us sometimes. No harm, no foul!