If you're 'unlucky' and you pause the game right before the timer is set to update, that update will be skipped. Do that too many times, and you have what appears to be a broken timer. Each game reacts a little differently, as well, so it's hard to predict. Your nursing moms will eventually recover on their own, and there's a workaround you can try that I linked to here, if you want to attempt an immediate fix.