And matter fact, did I mention that I physically feel great?
A doctor's approval is a waste of time, I know I'm straight

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

Meaning

Like many people with STDs, the woman in this verse doesn’t feel sick at all. But her choice of ignorance is dangerous. She chooses not to go to the doctor for a checkup or STD test and continues to prostitute.

The second line is also a double entendre; Keisha’s sister doesn’t need a doctor to tell her that she’s feeling healthy and that Dr. Dre’s co-sign does nothing to Kendrick’s music. He knows with or without Dre he kills it.

But her claim that she “feel[s] great” is a direct contradiction of her earlier claim about how tired she is. The line dismissing the importance of a doc’s opinion suggests that Keisha’s sister is in denial and is lying to herself in an attempt to make herself feel better. This is shown in subsequent lines where she tries to convince herself that she’s in control of her life (which is just another contradiction). Also, it implies that she doesn’t feel great emotionally/mentally.

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