Losing Weight

Hello everyone I am about to be 21 weeks pregnant and I have yet to gain any weight. In fact I have lost a totally of 8 lbs since I've been pregnant. My weight was fluncuating between 2-4 lbs so my doctor said it wasn't anything to worry about but in the last week and a half I've gone down a totally of 8 pounds. Is it normal? Is that something I should be worried about?