The pituitary gland in the brain produces oxytocin (also known as the anti-stress hormone and mothering hormones). It has many functions that include everything from increasing relaxation to causing muscle contractions and is even involved in orgasms. 
A similar reaction occurs while breastfeeding, as breastfeeding stimulates the release of oxytocin into your system that will cause the muscles in your breast to contract in what is called the Let-Down Reflex. During let-down, the milk-making glands in your breast contract to squeeze milk into the milk ducts, which contract to push breast milk through your breast, to your nipple, and into the baby’s mouth. As you breastfeed, Oxytocin continues to be released into your body – helping you produce your supply as your baby is at the breast.