What Causes Irregular Periods Post Child Delivery?

I am 24 years old and have had 2 children through cesarean section. It has been 2 years since my last surgery. Since giving birth to my second child, my periods have been absent for a long period of time, then they will come and last several months. I currently have been bleeding heavily for 2 months and it has not stopped. I am passing thick blood clots everyday as well. I am frustrated because I am only ever given birth control pills and sent on my way. Doctors think I am too young to have any serious conditions and dont bother looking into it any further. My question is: Should my condition be taken more seriously regardless of my age? What kinds of tests should I ask for? Thank you.