Well, over the week lots of hair starts to get loose, so when you decide to wash it, all this loose hair will come with it. Since you have thick hair, it's much more noticable. It really looks like you loose a lot, but remember that you may only be getting rid of 50 hairs a day with combing, while other losse hairs remain where they are and all get washed out when you decide to shampoo your hair.
Apart from that, always use a conditioner after you shampoo, use a wide-toothed wood or plastic comb, rinse your hair with cold water after you are done washing and rinsing it out, eat healthier and above all:
DO NOT BRUSH HAIR WHEN IT'S WET!!!!!
(seriously, this is a big no-no!)
Love,
Isa123