From Kidshealth.org:
"During the first few days to weeks after delivery, you may feel strong cramps in your uterus, a sign of the "let-down reflex" that means your milk has come in. Just after you start breastfeeding, you may feel a pins-and-needles or tingling sensation in your breasts, and milk might drip from the breast not being used.
Let-down also can occur if a feeding is overdue or before you start nursing (some women experience let-down from simply seeing their baby or hearing a baby cry). Or, it may happen after your baby is latched on and has sucked a few times (you may notice your baby starting to gulp). Some women have multiple let-downs during a single feeding.
Some women, however, never have a feeling of let-down, which is OK, too. Even if you don't feel it, you should still see milk coming from your nipple and hear your baby swallowing."