I gained 18kg with DS1 and lost all the 18kg within only a couple of weeks. I think it was the combination of the baby, lots of fluid and breastfeeding helped me burn off the extra fat. I know that i dropped weight really quickly because i went out and bought jeans the first week after the birth and by the end of the second week they were falling off me.

It might be worth checking in with the Dr though just to be sure.