It probably FEELS like you have lost half your hair - but my hairdresser tells me that this time of year is common for shedding loads of hair - kind of an annual moult!!!!!
My hair always comes out around now. Also - your hair looks long and so it WILL look like loads of hair!!!!
Its unlike to be CD that's causing it as hair loss from that doesn't usually happen unless you have been SSing for a long while - at least that's what I understand.
So probably its nothing to worry about - but if you are concerned why don't you go see your GP