That is how this pregnancy was for me. I became full very fast, ended up throwing it up, tried to eat again and threw up some more- and I lost about 20lbs. I didn't started gaining anything back until about 16 weeks, I had a itty bitty pooch, but nothing else! Don't worry the baby will be fine!
My advice try stuff high in vitamins- like oj with calcium, pre-natals and yogurt if you can.
Karlee