Not gaining weight ?!

I am 8 and 1/2 months pregnant and have gained absolutely no weight . My very first prenatal appointment I weight 161.2 and my appointment at 34 weeks I weighed 161. At my 20 week anatomy scan they told me I was having a big baby and that she was in the 86th percentile then at my 32 week appointment, my dr was doing an ultrasound because I had went to the hospital a few days before due to not feeling the baby move. Anyways on the ultrasound she noticed my baby's abdomen looked big so she did a measurement and it measured 36 weeks and 1 day. Then she said "her stomach is measuring 4 weeks ahead so let me measure her head" she did the measurement and it said 34 weeks so that's still 2 weeks ahead of how far along I am. She then measured my fundal height which was normal measuring at 32 weeks. 2 weeks later at 34 weeks I go in for another appointment and this time she didn't measure the baby she only measured my fundal height and said now I am measuring too small . She's sending me to a specialist Thursday to get better measurements and see how much baby weights but I was wondering if this has happened to anyone else or if anything knows what could be causing me not to gain any weight and for her to go from measuring really big to measuring too small .