It's either a combo of you possibly being pregnant or the fact you didn't ask her to be BM at your wedding.
Whatever it is, I'd speak to friend A first, ask for the reason why and then say about being hurt by being left out.
You could also write or ask to see other BMs and just put it on the line, ask if it's anything you've done etc, if necessary make them squirm. Or ask the most sympathetic one of the lot. Be prepared they'll pull ranks though which they already have done.
It is absolutely despicable that a 20 year friendship is in jeopardy over something bitchy or trivial, so I'd pull the "hurt" card.
You then have time after the above to decide what to do and if to end all these friendships and not go to the wedding.
Remember these so called "friends" sound a bit strange in the first place, fighting and then being friends after that, do you really want friends like that?! Also think of if you (not that you would) did similar to one of them, they'd be as hurt as you are now.
I also think you are quite possibly seen as "the nice one, who won't make a fuss" hence their treatment or maybe they for some reason see you being not as close to them.
For what it's worth I found that my own school friendships (which I assume these are) have mostly dissipated in the last few years, 3 of them completely fell out with for various reasons and different times over the years (got back in touch with 2 after long periods one not speaking and the other just not spoke since we were 15).
I am in touch with a few other school friends but it's very much arms length or as and when I choose to do so as we really have so little in common and 1 has been studying up north and the other lives in Leeds area anyway so rarely see them.
My current friends have known for approx 6-8 years are London based and we're fairly close. I must say though that also now (in past 3 years) I am bloody wary of becoming "best" or too close friends with people as its gone wrong in the past and I don't want it happening again!
Sounds awful and only one scenario really but I'd want it ironed out so I knew what had gone wrong.