The pain felt after childbirth is caused by the hormone called oxytocin. Oxytocin is released during breastfeeding and it causes the uterus to contract, which in turn helps the uterus to return to its pre-pregnant state.
The discomfort or pain caused by the contractions is called afterbirth pains. These pains are quite normal and there's no need for concern. In some instances, the increase in the number of births a woman experiences appears to be linked to more severe or stronger pain with each pregnancy.
Try breathing through the discomfort (big, deep, slow breaths in and longer outbreaths).