TheLastThneed, how is that like the OPs friend B? If, as with the OP, the other friends were extremely close and you were only good friends with one of them then yeah, you would be friend B and you wouldn't have been issued an invite on this occassion. But then if that was the case then, just like friend B here, you shouldn't be expecting them to always invite you and never have time together alone.
Do you mean that you were the friend that tied friend A and B together, who got shafted when they decided to get closer without you? In which case OP here is the one who tied A and B together, they aren't actually friends from the sounds of things. Not good friends anyhow, just a bit more than acquaintances.
Did you ever resolve it TheLastThneed? Or did they ditch you completely?
OP, just speak to friend B. If she's being PA then she sounds clingy and I'd wonder why she was trying to be included in everything that isn't really applicable to her. If she was joking then perhaps you need to ask why you think she's being PA? Do you not include her a lot and feel you should? Or does she sulk when she doesn't get what she wants? None of us know friend B, plenty of people -myself included- have been excluded and cut out. None of us know you either. Many people will empathise with friend B because they'll remember when they were left out, even if the situation was very different. Many will empathise with you, who have had clingy or PA friends. Hence why some will say YABU and some YANBU- most people have been on one and sometimes both sides and so will reply and put across their own feelings at being excluded or clung too.
Going on purely what you've put here, YADNBU. Whether this is a reoccurring trend, whether B feels left out, whether B is a cling-on...no one knows. Call her and see, she may have a different perspective that makes you think, she may laugh and say 'fuck, didn't mean it like that' or she might act passive aggressive.
Update and tell us?