Losing weight during pregnancy?

I'm nine weeks pregnant and the doctor told me i have hypermesis, I throw up constantly. Just last week I weighed about 131 lbs, but this week I'm sitting about 125 lbs. Kinda of a big drop in just a week. Not sure how much I should worry, Has anyone else been losing more weight than gaining in their first trimester? Any recommendations of what to do about losing weight?