Final answer:
Rebecca's crying postpartum is most likely due to a drop in estrogen and progesterone, leading to emotional lability commonly known as the 'baby blues.'
Step-by-step explanation:
The subject of your question is related to the emotional changes that can occur postpartum. Emotional lability, such as crying frequently after the birth of a baby, is most likely attributed to a decrease in the levels of reproductive hormones, including estrogen and progesterone, which drop dramatically after delivery.
This hormonal shift can result in a temporary mood condition known as the 'baby blues,' which affects many new mothers, leading to feelings of sadness, irritability, and emotional ups and downs.