A nigga behind me right now asking for ass and shit
And I'mma need that 40 dollars even if I gotta
Fuck, suck and swallow in the parking lot

from Kendrick Lamar – Sing About Me, I'm Dying of Thirst Lyrics on Rap Genius

Kendrick is retelling the story of how she had to sell her body for money. She had to get money by any means necessary and this was her way of making a living.

“The nigga behind” her is a play on “the future” that lies ahead of her from the previous line. Juxtaposing what impact her past will have on her future.

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