Irregular but didn't used to be?

I am 16, I started my first period around 13 and was completely regular with heavy flow, normal length, and about a 30 day cycle for over a year. Then I got irregular with lighter periods that came about every other month. Worried, I saw my doctor and got put on birth control for two months or so and had a light period both months. I didn't like the birth control though so I got off of it. Since then my periods have become extremely irregular. I had a short one in June and a slightly longer one in July. I remember I had huge clots and at one point went straight through a tampon in like 2 hours, this had never happened. I then didn't have a period until today, over 5 months later. I have not fluctuated in weight more than 5 pounds, my stress level has been higher these last few weeks than ever in my life so stress couldn't be preventing them, and I'm not sexually active. I exercise regularly and my weight is healthy. I know that being a teen these things can fluctuate, but this is extreme and I used to be regular, what could be causing this and what should I do?