Cycles getting longer

Britney

I’m a month shy of 22 years old and my cycles are getting longer each month. I started with regular 29 day cycles for the longest time and these past few months each cycle is longer. My last one was 34 days long. My period is still the same. I have one super heavy day followed by a few light days. I have bad cramps and heavy spotting when I ovulate. I also have hypothyroidism but have been taking medicine for it for almost a year. My levels were in the normal range in December when I had my last blood draw. I’ve talked with my doctor and she recommended starting birth control after I get my next period. Anyone know what this could be caused by?