My Doctor Told Me That I Had Gain A Lot

My doctor told me that I had gain a lot of weight during my pregnancy. I am not sure why that is because I have been eating completely healthy. I am now 30 weeks and it feels as if my son never stops moving. I am also extremely tired all the time. I also started showing by the time I was 3 months and I have had 3 ultrasounds. Is there anyway that it could have been a hidden baby?