What Causes Hypomenorrhoea Post Intake Of I Pill?

Hi, I am 36 yrs old, mother of a 9 yr old child. I never had any menstrual abnormality. Had a normal delivery. No history of oral contraceptive pills. My husband uses condoms.
I am having Hypomenorrhoea ever since i took I-Pill in January 2014.
Will it have any bad effect on my health in long run?
Do i need any treatment for this... my thyroid profile is perfectly fine. USG is ok too.