This has happened to me as well more than once. I've struggled with my weight my whole life (Polycystic ovarian syndrome, a thyroid condition, and an emotional eater). It happened a lot when I worked retail, from as early as 16.
Luckily, it's only happened once after college since working in an office environment. A few years ago, a woman I barely knew asked when I was due. I just gave her a blank look and said "oh, I'm just fat." She looked embarassed, but I just stared at her for a few more seconds (in case she wanted to apologize, not to embarass her) and walked away.
Somehwat related, I was overweight when I get pregnant (I'm 34 weeks along now). When I first started showing at 14-16 weeks and wearing maternity clothes, a woman in my building congratulated me on my good fortune. I thanked her, but then she said "so what are you, 7, 8 months along now?" I told her no, I was only XX weeks along. She then said "you look so much bigger than that. You look like you're almost done". I just looked at her and said "this is what fat people look like when they're pregnant. The fat doesn't go anywhere." She didn't apologize either.