What do you do when your best friend hates you?

My friend Katie and where friends that were like paper and glue, insepreable. We met at a volleyball camp and we’ve been friends ever since.
But later on in the school year, she started hating me. Well, meaning we started fighting more, she started having a “better best friend”, and her mom died of cancer. But what was worse was she treated me like crap, and still does to this day.
Ive been calling her and trying to figure things out and she gets annoyed by it. So do I, but she starts it. I really dont know what happend, she just started treating me like…shit.
I really need help, im even tearing up just writing this, I dont know what to do. Ive tried talking to my mom, and no help.