I have a 13yo and an almost 3yo and I suffered with heavy, painful periods after my 2nd child that lasted for an avg. of 10 days and I was getting it every 21 days instead of 28! The first two days were unbearable. I was not on any form of birth control since my husband had a vasectomy. I knew that my mom experienced similar issues that got worse as she aged and entered menopause so I went to my OB/GYN, which I suggest you do, and made sure first there were no causes for it as in my mother's case. I requested an internal and external ultrasound be done looking for cysts that could cause this type of bleeding.
I also wanted to know my options for changing this. Because I didn't need to be on the pill anymore and did not want more children, endometrial ablation was an option for me. This is an outpatient procedure, performed a couple of different ways, where the lining of the uterus is "removed" so that it will not build up, and thus, no more period! After researching, I chose the laser method which seems to have the least chance for ever having a period again (60% never do) and has the least painful recovery (just cramping the day of surgery). I had one period after it and then never again.
I think this is fairly common for periods to change after childbirth, but definitely not something we should have to suffer through. Go see your doctor and see what your options are. Good luck.