What is the mechanism that causes a loss of estrogen and vaginal dryness during breastfeeding?

ANSWER: The vagina consists of three layers: the vaginal epithelium, the lamina propria and the muscularis. All three layers have estrogen receptors. Estrogen is a major regulator of growth and function of vascular and nonvascular smooth muscle in the subepithelial layers of the vagina, the lamina propria and the muscularis. During menopause, due to substantial reduction in estrogen, a reduction in the thickness of the epithelium (from approximately eight to ten cell layers, to three to four) occurs. Estrogen deficiency influences the vaginal tissues, and causes the loss of collagen, loss of the ability to retain water and a thinning of the epithelial surface. Thinned tissue is more susceptible to irritation, and the vaginal surface becomes friable, with petechiae, ulcerations, and increased bleeding occurring after only minimal trauma.
During breastfeeding, women have high levels of prolactin, which exert an antagonistic action on estrogen production. For these reasons, hypoestrogenemia can occur throughout lactation. Decreased estrogen levels can cause urogenital atrophy, including epithelial thinning, decreased elasticity, and diminished vaginal blood flow. Symptomatic hypoestrogenic vulvovaginal changes are often referred to as atrophic vaginitis, and are typically present as vaginal dryness, itching, burning, irritation, and dyspareunia. Hypoestrogenemia may also cause urinary symptoms such as dysuria, urgency, and frequency.