Not eating and weight loss...

When should weight loss and lack of appetite during pregnancy become a concern? I am almost 19 weeks and I have lost 20 lbs since I found out I was pregnant. I'm still having sickness every morning, and some days throughout the whole day. I never really want much to eat, and I don't really even eat a normal amount of food most of the time. I'm hardly ever hungry. I go back to the dr next Wednesday