I haven't gotten any bigger

Ashley
So I'm currently 34.2 weeks pregnant and I haven't had an ultrasound since I was 24 weeks. 
I wondered how they know that my little one is gaining enough weight and such cause I have not gained a single pound. I have lost some and then haven't gained any back in weeks. Doctor always told me that that was fine (I am concidered over weight, and it wasn't an issue that I was loosing) I wasn't going to get another ultrasound with the doctor, but last Thursday when I went, he said that he was actually going to start me coming in for my weekly checkups, and an ultrasound next week because my stomach measured the same as it did 2 weeks ago. He didn't seem concerned. Heart beat was good and I feel her moving around everyday. 
But I didn't think to ask him if I should be concerned. Did this happen with anyone else? It's all I've been thinking about. I've never watched what I ate this whole pregnancy because I haven't gained weight, so now I'm constantly thinking about if I'm eating enough, and wondering if my little one is weighing enough and growing like she should be.