Unless your doctor expresses a concern about your weight gain (or lack of) things should be fine. Depending on what stage of pregnancy you're in there are or can be many contributing factors.
As for your second question, I personally lost 40 pounds under my pre-pregnancy weight after I delivered so for me it was simple. I had issues with what foods I could eat and was breastfeeding as well. Beyond this I didn't do or try to loose any weight. I suppose it would depend on each female and her own body. Some it sticks and others it melts off.
There are websites that show how many calories you need per trimester and how much you should gain. But in the first trimester 0-3 pounds and after that it's approx 4-5 pounds per month for the reminder of the nine months.
I gained 37 lbs during pregnancy (my son was 8lbs 3 ozs) and only 3 were during the first three months and I think I had gained about 10 lbs by 5 months...the rest were towards the end =)
As far as getting back into shape... I am back to my pre baby weight (only about 3 lbs left) my son is 4 months old. I fit into my old jeans and even though I run all the time my tummy is like jelly. Most say I look the same, but I have some work to do to get my abs back!