Getting Full Quickly

My OBGYN has told me that i need to gain at least 25-30 lbs because i am classified as underweight (im at 113 lbs) at 16 weeks, my past weight before pregnancy was 105 lbs and i am also 4'11. Whenever i do eat a large portion of food i get full very quickly, but before pregnancy i would've been able to eat everything. Is this something I need to speak with my doctor about or is this completely normal?