Oh my goodness.
I've had this happen too.
I had been working at a place for a long time, and about 3 other employees asked me if I was pregnant. I wasn't even seeing anyone, so you can imagine how flattered I was that they apparently thought so highly of me
(but I also worked in a town where such a thing was fairly common.)
When even the boss's son, who usually doesn't judge, asked me if I was pregnant, I finally put my foot down and asked why in the world people were asking me that.
One person said it was because they had seen me eating Saltine crackers (which I normally don't eat because they're just plain white flour), and, "Saltines are a pregnancy food."
Well, that one person apparently started telling other people... and suddenly, everyone just assumed I must be expecting.