The body has inbuilt mechanisms to try to maintain homeostasis (aka the natural body balance). It also has many feedback systems and can alert someone as to when levels of chemicals in the body are getting out of balance. This protective mechanism also applies to your reproductive hormones. When taking daily doses of synthetic hormones, the body registers that it is getting unusually high levels of oestrogen and progesterone throughout a monthly cycle. As the brain perceives an upset in hormone balance, it will try to correct any excess by shutting down production of natural oestrogen and progesterone. This shut-off may be why some women complain that their menstrual cycle takes years to return to normal after they come off the contraceptive pill.