you probly brushing too hard
If you are seeing an increase in loosing your hair I would speak to your doctor.... try and figure out what (if anything) you have changed in your life.
Your hair falls out as new strands grow... this is natural but it is different for each woman depending on age, hair type, etc. so it is hard to say what is normal for you.
I have thick, coarse hair and often clog the drain when I wash my hair (1-2 times a week in the winter) so I do not ever worry when I loose a lot of hair - I have a lot to loose and it grows so quickly that this is natural for me.