I'm 33 weeks and lost over 20 pounds in months 1-5 and have only gained 6 back so far. But my doctor isn't worried because buns is looking bigger than should be and he's healthy. That's all they really care about in weight is if baby is healthy. My doctor even told me it is okay to lose weight if I wanted as long as baby grows.
The same thing has happened to me. I lost 15 lbs from when I conceives to 13 weeks I went from 160 to 145. I had a doctor when I would have been 2 weeks and they told me it wasn't oregnant. And I lost 15 lbs and went to the doctor and made them test me again and they said I was and did a ultrasound and I was 13 weeks. I am now 21 weeks and I have only gained 3 lbs so I am now 148. Well my doctor said she isn't concerned and just to keep,doing what I am doing because my baby is healthy, I am healthy and everything looks good.
From your original weight in my first trimester i lost eight lbs and my Dr said to track from before weight loss cuz until you gain back the weight you lost your considered being under in weight gain they said its nothing to worry about though i freaked about losing though