weight gain

I am ten weeks and my weight is already up seven pounds. I started at a healthy weight and am now technically overweight.  The thing is, due to morning sickness, I have been eating less than usual. Granted I am not exercising right now, but there is no increase in calories to justify the gain. I am also extremely constipated and wonder if it just a lot of bloating and water weight. Anyone else gain a weird amount in their first trimester?