What Causes Severe Cramps During Menstruation?

i have been bleeding everyday for about 3 weeks now, like a period but fairly light and dark brown, sometimes very pale. i have been getting severe cramps as well, although this is not constant but usually i get them a few times a day. i am on the pill microgynon 30 and have been taking it everyday, and have been on it for 3 years. what is wrong with me??