That happened to me! It was crazy how one minute I would be okay and then the next I would be gagging because I was so hungry I wanted to throw up! I asked my doctor and he said it is very much normal. Your body can have a sudden drop on blood sugar and you have to feed it! LOL One day I was at Subway and by the time I finally got my food I was almost throwing up! It was horrible. Fortunately, after week 15 or so, around the same time my morning sickness went away, it stopped. I'm 38 weeks now and to this date I haven't gotten hungry like I did those first weeks!
If its that extreme I would contact your doctor and see if he could give you anything. If you can't keep a lot of stuff down it could be your bodies way of requesting nutrients.