Best friend said something really hurtful

So my friend A who’s also been good friends with our friend who passed away a handful of years ago, B, made a really hurtful post on her finsta about me. B and I were really good friends from 1st - 10th grade and then we drifted because she moved an hour away and transferred schools and then came back at the end of junior year and that’s when she passed away. And my freshman year of college i got a bid from a sorority her sister, C, is in and she chose me to be her little so obviously we’ve hung out a lot and stuff. Like going to bars and jet skiing etc and her mom and grandparents even inviting me over for dinner along with another girl from our sorority. Well yesterday her sister, C, called and asked if I’d go to the cemetery to see B cuz it marked 5 years and we took pictures with her and everyone there and i posted one to my story and then A made a finsta post that said like “how unethical and unmoral do you have to be to fabricate a friendship to get closer to a grieving family” so I’m like what the f*ck obviously that’s about me and i called her and she put me to voicemail after the second ring and unfollowed and deleted me off all social media and even deleted all our picture together. i don’t know what to do because she is one of my best friends, I’ve known her since i was a month old, and i don’t know why she would say something so hurtful if i never did anything wrong. And then she got two of her friends to comment on it saying how scum i am. What do i do? :( she did something similar to my other friend three years ago and put her on blast on Twitter and messaged the guy she was dating and told him how she’s a liar and stuff and all this false info about her