I think it was upsetting and insensitive. A horrible experience for you.
But I really can't think that this woman meant to upset you and I really object to the idea that she was attention seeking in any way.
Your pregnant belly which she cared about (as she gave up her seat) may have triggered this memory for her. A pregnant woman just like her own daughter (or DIL) who sadly lost her child. She may have just been speaking out loud. As many posters here have said; speaking of a dead baby within a family is often taboo or holds the fear that talking about the grief makes it worse or is somehow taking attention away from the bereaved parents whose pain will always be so much worse. I know that if I lost a child at birth, my Mum would hide her loss in trying to support me.
So it was a distressing situation for you and she shouldn't have put you in that situation. But I can't think she was selfish or attention seeking.
Wrong?. Yes. An awful person?. I don't think so.
And pregnancy is a particularly emotive and hormonal time but you will never, not ever for all the years forward from now not be utterly panicked and heartbroken by the idea that your child could ever be taken from you.
A random person on the bus talking about losing a child is distressing and upsetting. This feeling will never change because you imagine for a small second just how painful and life - altering that is.
And the odds are you will never experience that nor will your family and friends. But people do experience the terrible loss of their children dying every day. The numbers are small but still occur.
And the fact that no-one unless they've been through it; can really imagine how destroying that would be and not ever want to imagine it would happen to them is what leads bereaved parents to feel so isolated and abandoned. They don't want to remind other people of how life can change in a second.