Thanks for the query.
Menstrual cycles can undergo changes due to many causes like:
Weight changes, stress
and strain, recent illness etc affecting hypothalamo-pituitary axis.
Poly cystic ovary disease
So you better consult gynecologist, get examined and go for ultrasound
With this the possible cause for your problem can be made out and treatment can be given accordingly.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429