Hi - try not to worry about losing the weight at this stage. I have learnt a lot of it (becasue its not fat) drops off. Being left with bettwen 1/2 - 1 stone. At end which then comes off if breast feeding, its your little store for the bubs food.
I have put on a lot although only notices on the scales. I put on 7 pound over my first 2 couple of weeks pregnant. Eat as normally as you can albeit the sickness. I wouldn't try to get into the mindset of dieting, the baby needs all the nutirents it can get.
I was worried for health reasons and whether there was something wrong, Im in my second trimester and way over what they say is 'normal' I was lean, smalll to start with. The smaller you are the greater weight gain it would seem.
I think in total I will put on between 3-4 stone, have a hunch after plasma blood drop etc, I will be 3/4 of stone heavier than at begining of preganancy. Which for me doesnt bother me in the slightest though because If I have a healthy weight baby I will be over the moon. Babies need you to eat protein, breaking down your own muscle in order that the baby receives its protein that way (under eating on protein) is not the same as eating the food yourself, this could change the metabolism and the balance of your babies on system for good. Remember it adapts is body, its blueprint to the world/diet it thinks its growing up into.
Having said that you are early days so don't beat yourself up at this stage if you just cannot eat very much, do whatever you can. The insurance poilicy of a preg multi vit can help to rely your fears a bit too.
To conclude, I haven't over eaten so I have now got accostumed to thinking its obviously what my baby feels it needs to do to survive. For me to put on 0.5 stone straight away, not eatign very much (mornng sickness) must be the name of teh game for me. Opposite to what I was expecting because I was small I thought I would put on very little but its all the blood volume and everything else.
Goodluck