Both pregnancies I've lost weight in the first trimester (15 pounds first one, 10 pounds this one)... it's totally normal! Your doctor or midwife would let you know if it's reason to worry. :)
Your body was made to do this. If you don't gain anything early on, baby will get resources from stored fat the human body already has. Later on you'll gain weight and (hopefully) not experience morning sickness throughout. I started gaining my 4-5th month when I was able to hold down a meal.