What Causes Continuous And Heavy Bleeding After Childbirth?

i only get a period once or twice a year,since having my second child 14 mo. ago i have had at least 4 very heavy periods. i had a period begining Dec.5th. Then again Jan.27, and am still bleeding with blood clots,no cramps. whats going on? i am not on birth control and i breast feed during the night,