Maybe because you are still eating like you were pregnant. I am breastfeeding and slowly going back to pre-pregnancy weight. I am watching what I eat and walk up and down the stairs a few times a day.
You only need 350 extra calories a day while breastfeeding. If you are not losing weight, then you need to reduce the amount of calories you are eating a day.
Women take food and medicines rich in vitamins after delivery. In some places women take rest for a long time eating medicines and food prepared in ghee. Apart from this, in your peculiar case, I feel you are extremely happy by the birth of a child and become fatty.
women gain weight during pregnancy around the hip region so that they can have an easy delivery. After deliver the baby feeds on the mother's milk and takes in the required nutrients from the breasts. Losing weight has got nothing to do with breatfeeding.