Help have not gained

I’m starting to worry I have not gained a single pound. I never had issues with me weight or being too skinny. I also have never had issues with not gaining weight I would gain the 5-10 lbs randomly here and there and it would fluctuate back to my usual weight. I’m still at my pre baby weight. I’m 20 weeks 4 days and the baby is measuring a couple days smaller than she should. The ultrasound tech didn’t have any worries but why am I not gaining! Help? What are some foods to help with weight gain?