I usually start getting white pasty clumpy cottage cheese like discharge a week before my periods,and i have painful sex during these days.

Vaginitis tests: are indicated. It could be that you are keeping a low level infection that flares up during the week prior to your period. The symptoms you describe are consistent with Candida vaginitis (yeast). It is a good idea to be tested prior to treating vaginitis, because symptoms of the different types can overlap and incorrect treatment can make symptoms worse.