If she is overweight, then the question may have played on those insecurities. If she isn't, well, it may also have played on her insecurities. Women can be insecure like that.
However I am not sure Jetstar deserves that much of a hard time.
Apparently hundreds of women have posted sympathetic messages on social media, with some abusing the airline for this incident and others like it. One posted "That is absolute crap from Jetstar!"
It's not like the attendant was asking to be malicious or intentionally humiliate the woman. He was just doing his job, as women more than 28 weeks pregnant must carry a letter from a doctor or midwife on Jetstar flights for safety reasons.
Yes it may have been extremely poor judgement on his part - for which he no doubt felt terrible - but is being wrongly asked you're pregnant really such a massive insult?
Sometimes we need to keep things in perspective, realise humans are human and have the self-confidence in ourselves to not let another's mistake "humiliate" us.
I like to think in that situation I would smile, hold my head high and, as a wise young pop singer once said, 'shake it off'. While swiftly pocketing my $100 voucher of course.