Grief can be strange in who you do and don't want to tell about something, or when. She might have felt like she would jinx having a healthy living child (you) if she told you about what it took to get you, or she might have never felt right speaking about it before but time managed to heal some of the hurt, or she felt that speaking about it now was more important than keeping quiet (depending on who/how/why she told on Facebook). I understand that this was a massive shock for you to hear out of nowhere, but she almost certainly had a reason for not telling you about her experience. Ask her and see if she is willing/able to share it now.

She never owed you that information. That's her personal business.

it's easier to talk to strangers sometimes. to get it off your chest and not talk to that person again.

It's not about you versus strangers. She finally went from feeling like she couldn't talk about it, to feeling like she could (and ought to). That's a good thing. People like your mom have felt really alone in their experience, because nobody used to talk about it, so they all felt like they were the only ones. The fact that she's feeling more free and comfortable to talk about it is a great sign for her, and she's also doing good for others.