Why is my hair falling out when I wash it? 5y cancer free history of neuroblastoma, ptsd, depression

I don't know why. Dr stuart gave a great answer. In addition to those medical causes, stress associated with ptsd or depression could also be a factor. Here is a short article by the mayo clinic on impact of stress with hair loss: http://www.Mayoclinic.Com/health/stress-and-hair-loss/an01442. I agree that this would require medical work up. Take care.
Get some help. Hair loss can be due to many different causes such as hormonal problems, endocrine diseases, skin conditions, auto-immune conditions, and drugs, for example. You should see your doctor about this.