lossing and lost

I keep track of my weight extremely close. I am currently pregnant with my amazing baby number 2! With baby number 1 I only gained 8.3 pounds in my whole pregnancy my doctors were really concerned but i gave birth to a beautiful healthy baby boy at 39 weeks. I am currently 9 weeks and have dropped 7 pounds in 48 hours! Any other moms go threw this? I am so lost and confused. I eat 3 meals a day and 2 snacks plus lots of fluids and I still cant gain weight. Since I found out i was pregnant i have lost a total of 18 pounds. before i gpt pregnant i was at a healthy weight almost considered underweight and now i am really underweight. please give me some tips or tricks! so lost!!