What Causes Prolonged Periods With Abdomen Cramps After Giving Birth To A Baby?

hi, i am 28 years old and have one child who was born naturally on june 17, 2009. my period has been normal since his birth up until now. i am also on the pill. my period should have gone away on saturday when i started my new pack of pills but it didnt. it actually got extremely heavy and i have a terrible headache and slight stomach cramps. i am having large blood clots as well. any ideas on what this could be?