Uh oh. Okay, Should i be worried???!?!?

Im 21 weeks pregnant and everything that i read says i should have gained 10 to 15 pounds by now, But since i cant eat much without throwing it up I haven't gained any weight. Ive actually lost weight. And im really worried that my appetite isn't going to change and my baby will be under weight. What should i do? Ive already tried the crackers and the ginger ale and all that. I need something thats actually going to help.