Trouble Gaining weight

I am 17weeks3days pregnant with my second child, I had all day sickness from 6 weeks to around 14 weeks, and lost about 20lbs. I was 135 before pregnancy. I've been eating like crazy and I feel way better than I did. But I can't seem to gain any weight and its really worrying me. My last ultrasound at 12 weeks, my doc said she looks great. I go to my 18 week ultrasound on the 6th. I just want to know if any one else had the same problem with gaining weight. ? And what did u do for it ?