Could be from hormones, could be from almost anything really. I lost fifteen pounds in my first trimester but that was because I was really sick and couldn't keep food down. Don't worry about weight, your doctor will tell you if need to gain more or less if they are concerned. But every pregnancy is different. I've only gained back four pounds of those fifteen and I just started my third trimester. My doc says that's fine because I already have extra weight and baby is healthy. Just eat smart and exercise smart and you'll be okay :)