What did Padmé die of?

According to Episode III:

MEDICAL DROID: Medically, she is completely healthy. For reasons we can't explain, we are losing her.

OBI-WAN: She's dying?

MEDICAL DROID: We don't know why. She has lost the will to live. We need to operate quickly if we are to save the babies.

_{^{Revenge of the Sith : Script}}

Medically healthy humans don't die for no good reason.

So what did Padmé die of, and how did they even know she was dying?