It isn't unusual to lose some weight in the first trimester. As the previous posters said, just keep eating a good, balanced diet, take your prenatal vitamin AND drink lots of water and you will be fine :)
I've lost 6 lbs over the last 2 weeks, my doctorsays its normal in the first trimester.
Like you I lost weight in the beginning. As long as you are eating well, you shouldn't worry about it. Make sure you take a pre-natal vitamin to ensure your little one gets all the nutrients they need. I only gained 15lbs and that was in the last two months. I don't think you need to worry about it unless you are vomiting everything you eat.
Good luck!