Weight gain?

Sierra
I had my anatomy scan Tuesday and everything looked great. The doctors told me he was 2 days behind though and at the 14th percentile. I feel as if it's my fault considering I haven't been able to take my prenatals as they have made me sick, and eatting was impossible the first 3 months I was pregnant. I lost 5lbs total and they told me that I needed to gain more weight. I gained 6 back but obviously that's not enough. Pre pregnancy I weighed 107lbs. I've always been 107 for as long as I can remember and I remeber trying to gain weight was a struggle. I know I'm needing to gain weight but I have no clue how other than to eat fatty foods 😔 have any of you other lady's had this problem? And if so what did you do?????