I'm scared. VERY low BMI. think I'm severely sick

Had a midwife app today. My height is 5ft 6. I am slim fit. Today the midwife checked my BMI and it is 17 😭😭 I am officially classed as underweight. I am a normal 20 year old woman, with a child pregnant again. It scares me so much because I have had anerexia problems when I was a child. 
•I can't seem to put any weight on. At all! I eat 4 meals a day spaced out through the day, healthy meals. And snacks between. I do this 7 days a week. 
•I have lost 5 stone since the 11th of December. But eating so much, regularly. I'm worried sick because I'm pregnant, already diagnosed with hypermesis. I'm worried what this will do to my baby. 
Please can someone help 😭