What Causes Hair Thinning?

Hi, For the past 4 months my hair has been thinning..I have probably lost at least half...I have been to my family doctor and dermatologist...and they only thing they did was give me xanax so I wouldn't stress about it and b12 shots once a month. I do not know what do do and what is causing this. My Mom and 4 sisters all have a full thick head of hair, as I used to too...so its nothing that runs in my family. Thank you for your help. Cindy