What Causes Hair Loss After Washing Hair?

Hi, every time I wash my hair (which is about every 2-3 days) I am losing a hand full of hair...every time. This has been happening for about 5-6 months now.

I used to have really thick hair and now its all thin and my scalp is so itchy.

I dont use any products on my hair and I colour it to match my natural colour every 3 months. I thought it might have been the brand of colour but I have used the same one for many many years now. Do you think you could tell me what could be wrong?

Many Thanks

Carolyne