so what is girl A's opinion of you now... do you call and she doesn't pick up? You just need to confront her in person... tell her how much she means to you blah blah blah and that B is not at all a part of your life.
Oh and DELETE YOUR FACEBOOK ACCOUNT; girls love snooping on that **** to see who is writing what on people's pages. Girl A also stepped over the line though by messaging B right off the bat... she should have asked you about it. A doing that is pretty creepy honestly, but I think it also shows how much she is into you... it was an emotional move on her part