Need some input.

My midwife is mildly concerned about my weight. Not that I'm gaining too much, but that I haven't gained any. I am four months pregnant and only weigh 145lbs. I weighed 158lbs. the day I tested positive at home. Morning sickness has been kicking my ass so bad. I can't just sit here and make myself eat, the chances of it staying down are nonexistent. Just need some back stories on if anyone else has dealt with this problem? I didn't with my first, I gained 40lbs. my whole pregnancy with my little boy. I'm worried this may end up hurting my baby but I'm not sure what I can do.