Very sudden weight gain in early pregnancy?

I am about 7-9 weeks pregnant(LMP is nearly 9 weeks but dating scan is 7.5 weeks), anyway I have already gained 3 kilos since I have found out I am pregnant and I just don't understand how this possible while I am still breastfeeding and my intake hasn't increased more then 200 calories( I calorie count and increase from 1200 to 1350 to sustain breastfeeding and pregnancy).