A Completely Inverse Weight Problem

So, an ex-boyfriend once described my inability to gain or keep on weight as "a completely inverse weight problem." I have always been small and never registered on the pediatricians’ growth chart, but I was always told I was the right height for my weight. Proportionate. I won't tell you how much I weighed when I got pregnant, but I will tell you I'd been the same weight for about 10yrs at that point.
My pregnancy books and a midwife had told me a pound a week was the goal during the first trimester. Well, given my morning sickness and fatigue what sounded completely reasonable was quite impossible. I figured I'd make up for it eventually (or at least I hoped so). The doctor never said a word about it until my second trimester started and with the sudden relief of morning sickness I mysteriously started losing weight. I was mystified until the doctor deduced that my activity level was canceling out my calorie intake. Oops, forgot about that high metabolism. I was burning off those 5 meals a day.
Well, things slowed down quite a bit when the contractions hit because that lead the way for pelvic rest and eventually bed rest. I was now eating all day and not moving around at all. Voila! I have gained steady weight recently and since the start of my pregnancy I'm up 20lbs.
Now, even I worry about losing the pregnancy weight once the baby comes. Partly out of vanity, but it also reeks havoc on my joints and balance. It was one of the motivating factors for doing maternity yoga (second to pain control). Well, I read in my pregnancy book the other day that you burn 500-650 calories a day breast feeding. Great, I'll have a newborn and be exhausted and not eating as much as I'm burning and drop the weight too fast or drop so much weight milk production stops. I know it sounds like a wonderful problem to have, but it's gotta be similar (albeit more socially acceptable)to being overweight. Both situations can cause health problems. Anyway, not one to be caught unprepared I've been doing research and added the question to my list to ask the doctor tomorrow. I think the best solution I've heard so far is to eat every time you breast feed or pump to replace some calories. I'm open though, if anybody has suggestions let me know.