When you have your first baby your uterus is toned and more 'solid' than with later babies. The more babies you have, the more "lax" your uterus is.
After baby, your uterus continues to contract for days afterwards to shrink it back to its original size. With your first baby or two, you dont really feel these contractions. The more babies you have, the more you feel them. Can be extremely painful and are much worse if you are breastfeeding. Just like nipple stimulation can bring on contractions to put you into labor, breastfeeding causes your uterus to contract after birth as well.

I have recently had baby #3 and never, ever in my life have i experienced such pain - and it was defn worse when breastfeeding. Its enough to put you off having another......