I know this is an older post, but I am just now reading it. I have two things to say.
1. I did that once. I asked a cashier at Wal-Mart when she was due when I was pg with Bryce. She, like you, simply said "I am not pregnant!" I felt mortified. I will NEVER forget it and I will never do it again! Now, someone has to actually tell me "I AM PREGNANT" before I will even assume they are.

I am sure that hurt your feelings like nothing else, but please know that I am sure she was very upset by having asked you that too. Especially since she knows you and sees you regularly.
2. I gain weight in my belly. My mom is 58 and looks 9 mos pregnant. I have also been asked if I was pg - but it was right after I had my youngest son. I fully know the struggle that you are talking about. I have gotten so depressed over my weight before. I yo yo too. I was 105 pounds (5' 6" tall) when I got pg with Bryce. At my biggest, I have been 179 pounds and NOT pregnant! Right now, I am about 145 or so. I really go up and down and it really sucks!
Keep your head up and know that you are a beautiful girl inside and out. I have not seen your whole body, but in your siggy, you have a beautiful face - with a bright and happy smile. And you have a husband who loves and adores you. And friends here...when you need us!
((((hug))))