Sheryl Crow -- CHOKES On Stage ... 'My Brain Has Gone to S**t!'

You can't accuse Sheryl Crow of lip-synching, but you can say her memory sucks -- because she flat out forgot the lyrics to one of her biggest hits during a concert.

Sheryl's epic flub went down during her Saturday night gig in St. Petersburg, FL. Crow was bringing down the house with "Soak Up the Sun" ... when she suddenly brain farted.

In the video ... Crow trips over the lyrics, but like a pro, she keeps going and asks the crowd ... "Oh, what's the words?" Then adds, "It's live! Nothing on tape here!"

Crow smiled and gave the awesome excuse, "I'm 50 what can I say! My brain has gone to s**t!"