It is normal to loose weight in the first half of preg. I lost weight both times and didn't start again until after the half way mark, so far 3kgs and I have not got much of an appetite, so I do my very best to make sure what I do have is good food, ie if I really don't want to eat I try to have a banana smoothy on skim insted (I can't have full fat milk).
If you've got enough weight on your bod there isn't a real concern with loosing weight during preg. I really wouldn't worry too much, just see what your Dr says at your next appointment.
Good food is always the best way to go, especially if you can't bare the horse tabs they call 'preg multivitemins' (I was just the same, the last thing you want to try when your throwing up all the time is to shove a big tab down your throat in the hope it doesn't come back up!)!