Expecting a Faux Pas
I thought everyone on the planet got the memo that you never, ever ask a woman if she's expecting, and yet, believe it or not, it happened to me. I was at a family picnic with my husband's side of the family. An older woman who I'd never met before pointed to my midsection, perhaps still a little flabby from being pregnant two years ago, and asked if my daughter was excited about becoming a big sister. I stammered something about not being comfortable answering that question and walked away, but I'm tempted to follow up with her. She's not a close relative and I may never see her again, but even so, I want her to know how unacceptable this comment is! What, if anything, can I say to her after the fact?
Expecting a Faux Pas
I, too, thought everyone received that memo, but perhaps some people forgot to read it or thought it didn't apply to them or any number of other excuses that should all be banished because really, you're right, you never ask this. Ever. It doesn't matter how long the woman has been married, how old her youngest child is or how much you're sure her shape is different from the last time you saw her. Ranting aside, it sounds like you handled the situation as well as could be expected when you were caught off-guard by an inappropriate comment.
Now, though, I'm sure you've had plenty of time to stew in your not-at-all pregnant juices about the things you wish you had said. Save yourself the path of regret and realize that none of those things would have been productive. Even though you think that having said something more direct and confrontational would make you feel better now, you may have ended up with other regrets, like insulting an older relative of your in-laws. Besides, the real insult (if you're anything like me) comes from knowing that someone was looking at your body in a judgey way. Once she said it, the damage was done, and no retort on your part would undo that.
The best thing you can do is put this incident behind you. Throw away the shirt you were wearing if you must (yes, I've done that). Remember that your body did an awesome thing by giving birth to your daughter, and no insensitive comment can undermine that. Don't shirk away from social obligations with your husband's family. Don't shrink in embarrassment if anyone mentions the incident. Resist the urge to say that the relative was obviously malicious/senile/blind.
As much as I advise you not to say anything, you also don't want this relative to go spreading the "news," so you may want to do some damage control. Perhaps you could have your husband talk to his parents and put them in charge of quashing this rumor before it goes anywhere. If that conversation is too uncomfortable to contemplate, ask him to send an email to the tune of, "Can you believe Great-Aunt Mildred asked Diane if she was pregnant? Please make sure she knows that she's not before she tells anyone else in the family. While you're at it, make sure she knows never to ask anyone else such an inappropriate question! For the record, I think Diane looks as amazing now as she ever has!" OK, maybe you don't need your husband to say that last line to his parents, but make sure he says it to you!