I suspect it's the Lochia. After giving birth a woman has a LOT of fluid to get rid of in only a short amount of time. I never had a cesarean, but I suspect it's similar to vaginal birth in that your body needs to slough off the fluids that held the placenta. That usually takes place over the few weeks following giving birth.

CLARK: No. I'm just worried I'm a jinx.
JONATHAN: A jinx?
CLARK: Yeah. Let's face it, ever since she's known me, Lois's been kidnapped, frozen, pushed off buildings, almost stabbed, poisoned, buried alive and who knows what else, and it's all because of me.
-"Contact" (You're not her jinx, you're her blessing.)