You sound like me, I always worry about my weight. You aren't overweight though as the calculation hasn't allowed for the fact you are pregnant, just like it doesn't allow for people wth more muscle which is one of the flaws with the BMI. I think I read in the Good Weekend a few years ago George Clooney and Brad Pitt were considered overweight purely based on their BMI because they were heavier from their muscle bulk.

You will know if you really are overweight or not by other parts of the body and if I were you I would scrap the BMI calculations until well after you have bubs.