too skinny?

I'm 18 weeks pregnant and haven't gained a pound. Baby is growing fine and normal, my belly is still very small. Almost no bump at all. And at my doctors appointment today, I found out I weigh the same as I have for the last 3 months! I just don't have a huge appetite. I get hungry and eat when I am, but my appetite has severely decreased since I got pregnant. Is that normal? Or am I doing something wrong?