Sandra Denton, aka Pepa, is suing Dr. David Sayah for a butt job gone bad.

It started out good enough.  Another doctor had shot biopolymer injections into her butt and hips to give her a bigger thang to shake.

Unfortunately, The Salt-N-Pepa rapper got into a car crash in 2018 and that messed her injections up.  She was referred to Dr. Sayah, who she says convinced her to replace the injections with smaller ones because he was worried about her psychological reaction to suddenly having a much less bootylicious backside.

Pep -- who is anywhere from 51 to 56, sources vary -- agreed. But that led to a series of botched surgeries and liposuction which left her butt hardened and painful, making it difficult for her to perform.

She finally had all of the implants removed.  But the pain still persisted and when she went to another doctor he found a "large amount of biopolymer silicone and dead scar tissue remaining."

So she's suing Sayah for disfigurement, arguing she will likely suffer injuries for the rest of her life.