I know the obvious answer is, "because you just had a baby!" But what is it about that that makes you feel so weak?

I was in decent shape through my pregnancy. Nothing spectacular, but I did yoga and went hiking at 34 weeks. I regularly went on long walks with DH and LO. But then you give birth and for the first week, walking across my house was tiring! Today, at 4 weeks postpartum, my mom and the LOs and I went on a walk around a pond...probably about a mile. And I feel like hell. Sooo weak and worn out.

Is it birth? Is it the recovery from pregnancy? Is it the fluid loss? Something about hormones? The sleep deprivation? Something else?

I'm really curious. I mean, I get why your perineum would be sore, but why is just walking so hard? Especially when I could do it pregnant!