I know exactly how you feel. I went through the exact same thing. It lasted about 3-4 weeks but then my appetite returned so I wouldn't be concerned. My doctor said that it was normal.
hey,
i'm a few days away from 18 weeks myself and i certainly have noticed a decrease in appetite too. not that that is terrible cause i don't want to gain much this pregnancy but i understand the concern. i make myself eat something each meal cause i know its important, i just don't eat a lot.
your body will tend to babies needs first so don't worry about appetite too much. your body will let you know when it needs food.