Hi mickey
I had/have the exact same problem as you. I lost a stone in 4 weeks BF my two. Very scary at the time and I found that I didn't get an awful lot of sympathy or understanding. It seemed to stabilise out at a stone under what I was pre-PG which I think is far too thin for me (size 8 and 8 3/4 stone)
I stopped BF Charlotte nearly 3 months ago and Tom nearly 2 months ago but I've only managed to put on a few pounds so far. Up to 9 stone.
I ate as much as I possibly could while I was BF even taking bananas to bed with me for a snack in the night cos that BF made me hungry! LOL! HV suggested making myself some calorific smoothies but I didn't really have the time to faff with that. I found it hard though because I have a healthy diet and it's difficult to change your ways to remember to eat biscuits or something to add easy calories.
I weigh myself when I take the children to the clinic to be weighed to keep an eye on things. I felt that way the HV also started to take me seriously when I insisted each time on being weighed.
Good luck. Keep your eye on your weight.
