Lizzo says her new weight loss is gonna have people asking if she got a Brazilian Butt Lift ... and, she's says TMZ had better investigate to put the rumors to bed.

The singer-songwriter posted a clip to Instagram yesterday ... stunning in a red lingerie top and a pair of matching see-through pants -- which did little to hide the underwear underneath.

Lizzo shakes her booty a bit ... and, says the whole look is "giving BBL" -- meaning she's looking like she went under the knife to get a backside like that.

She's laughing -- clearly making a joke ... before adding everyone online is going to accuse her of getting the surgery, and she actually shouts us out.

Lizzo says everyone's going to come running to TMZ to find out if she got a BBL ... and, while we don't know for sure, we'd venture a guess and say her behind is sculpted from months of working out and eating right.

We've told you all about Lizzo's weight loss ... revealing she hit her fitness goals earlier this year. She's been known to flaunt her figure in her videos -- including one she posted late last month which showed her twerking in a pair of jeans.

We don't think Lizzo got surgery ... but, hey we're happy to look into if it'll make the star happy!