Why aren't I gaining any weight!?

So I'm now almost 19 weeks pregnant and haven't gained a thing, my bump seems to be quite small too, women around the same weeks as me seem a lot bigger bellied by now. Not only have I not gained anything I've actually lost 11lb... I wasn't overweight in the first place and was a lot happier with my body before I lost it all, but no matter what I try I can't gain it back. My weight now is 127lb and I'm 5'7... When should I expect some weight gain as I'm almost half way now and expected to atleast gain back my loss :(