Microgynon is an monophasic oral contraceptive. Large lumps of tissues and painful periods are not side effects of any oral contraceptive. These large lumps of tissue are endometrial tissue which are being shed in order to start preparation for the next cycle. Blood contain an clotting factor to reduce blood loss while uterus produce anti clotting factor for smooth secretion of blood from uterus. Sometimes, this anti clotting factor is not produce in sufficient quantities causing clots or lumps of tissues. This condition also cause cramps. So, you should not worry about your condition. It will be OK after few cycles.