Sorry lots of questions!

I am 5'6 and I originally weighed 110 pounds and I am 19. I am now pregnant with twins and I weigh 126 pounds. (almost 5 months)
So after my twins are born how much do you think I will weigh?
And after they are born can I start going back to exercising and my original eating habits while I am still breast feeding?

My weight is around 110. My doctor says I am and that is underweight.
Is it really unhealthy to weigh this much? I can never hold the weight and my bf always worries about my weight. What do you think is wrong?
I am not an bulimic or anorexic so can you keep the nasty comments to yourself!