Weight loss at 31 weeks?

Hi ladies I'm almost 31 weeks and I was 168 prepregnancy and I have lost down to 158 then started to gain back up to 164 during 2nd trimester where my weight was maintained for a few weeks but now the last month I started losing weight again and am down to 158.2. Has anyone gone through this? My Dr said we will keep an eye on it and wait and see. I thought I was doing better eating more and I have no nausea or anything. I've been peeing a lot but that's been normal with my other pregnancies and I've never had any issues gaining weight before.