The same thing happened to me, but luckily he started going on about his pregnant daughter so I didn't have to lie. I went straight on a diet after thatI got asked when I was due even before I fell pregnant because I'm a bigger girl. Rather than get offended, I'd say in 3 months and then walk off.
I didn't want to have the awkwardness of saying "I'm not pregnant" then feeling bad for making them feel bad for saying something quite innocent.
At least now though, if and when someone asks, I can say "Due in May"