Thanks for the query.
some amount of irregularity in the flow and duration of menstruation
But as you are having heavy flow with clots, better to get investigated once to rule out the possibility of rare causes.
Consult gynecologist once, get examined and go for ultrasound
If increased endometrial thickness is noted, she may advice endometrial biopsy
With this workup the possible cause for your problem can be identified and treated.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429