I had this same problem. I saw a dermatologist, she asked if I started taking new medication or if I have resently been under a lot of stress, my answer was yes she checked the roots of my hair and could tell when the problem started.
Her cure was to use the men's formula of rogaine. Within a few months my hair was growing in thick and I no longer had hand fulls of hair in my hand brush or comb.
Hope this helps for you!