What Can Cause Irregular Periods?

Hi. I am turning 18 next month. Ever since my family moved from Brazil to Canada, last 2008 I have been experiencing irregular periods. I have only a few periods a year. Before I came here, my periods were completely normal. I went to Brazil 2009 and this year to visit, and I had my periods back again. For the past 3 years that I have moved here, I gained 15-20 pounds. Earlier this year I went to our family doctor regarding my problem. She gave me Ethinyl Estradiol and I bought 4 packs. I finished the packs last June. They worked perfectly fine for me, but when I finished my pack, the next month, my period stopped again. What could be the possible cause of this? I am extremely terrified.