My best friend is mad at me with no reason?

Yesterday i have received a message from my best friend 'Please don't think that I'm stupid'. I knew something is wrong because she never talks to me like this unless she is upset or whatever, so i asked what happened? i was really scared that it was me so i called her for a several times but she didn't pick up her phone. Then i sent her a message 'whatever i did i'm sorry', because similar stuff has happened to me before and i don't want to lose my best friend AGAIN so i thought i should apologize first. then she says 'what have you done? why are you saying sorry to me?' well apparently she thinks i KNEW i did something wrong and now she found out so i am apologizing but no, i have no idea what was going on. i kept asking her why are you mad at me, what happened, is it my problem can you tell me what i have done etc etc but she keeps telling me something indirectly and i can't get her meaning at all. then she says 'I'm so stupid, I dont' even realize people are telling lies until the last minute. I'm so naive.' and 'leave me alone'
later, i saw her posting a picture on instagram saying that someone has betrayed her twice and she had had enough of it and i'm doubting if she is mentioning about me.
what should i do? i've called her for many times but she cuts me off everytime. I can't always be the one apologizing, i don't even know what happened. i've asked her friends(which are also my friends because we are in the same group) but none of them seem to know what happened either. i can't talk to her face to face because she walks away from me whenever she sees me. i care about the relationship a lot and i don't want to lose someone in my life again.
what should i do?

added:
actually earlier that day i had gone out with friendA who my bestfriend doesn't really like, to have a long cycle ride. there were only two of us in the beginning but Another friend in our group was invited by friendA and i had no idea until the day before. i guess my best friend is mad because she wasn't invited? we had had a small misunderstanding before(i thought my bestfriend was mad at me but she actually wasn't), is she considering the misunderstand as a first "betrayal" from me as she says she was betrayed twice on instagram? i'm really confused and worried. i really hope i could know what happened