So let's start with The Diss -- Comedian Mike Epps and the group Mindless Behavior were co-presenting the award for Best R&B Artist when Mike jokingly tried to sing.

One of the members of the group quipped, "You sing BAD ... like Lauryn Hill's tax accountant bad."

The diss drew gasps from the crowd ... and angered Beyonce, who ultimately WON the award that was being presented.

But TMZ has learned ... B was so upset, she scrapped the speech she had prepared and defended Lauryn's honor instead ... saying, "I want to thank all the talented R&B women that came before me like Lauryn Hill and Mary J. Blige."

When B walked off stage, there was a woman waiting for her -- possibly a BET handler -- and B began to vent about the insult, saying the Lauryn Hill diss was "disrespectful" and "not cool."