I lost weight for a while too. Dr always said my weight was fine. I've been gaining a little back the last few weeks, but still not a lot. I wouldn't worry about it too much :)
I lost 3lbs for not eating for 17hrs but it wasn't my fault.
It's not safe to try to lose weight during pregnancy. During the 3rd trimester you're supposed to consume about 400 more calories a day than you normally would and this is the time that the baby is rapidly gaining weight, so most women gain weight too in the 3rd trimester. It's good to be healthy but you shouldn't be trying to lose weight, not saying that you are.