What Causes Irregular Periods After Discontinuing Birth Control Pills?

I came off the pill 13 months ago. I had been on the pill for over 10 years prior. My gp originally put me onto the pill when I was 16 to help regulate my period. No issues with cycle over the years. ISince coming off the pill my cycle has been irregular and lasting 5-7 days. The dates range from 30-81 days in the cycle. The flow is light with little to no cramps. I have recently had an ultrasound the dr looking for pco syndrome. Nothing there to rule it in or out. I thought I had started my period the other day. It is dark brown and very stringy. This has happened before but it has been like that for the last 2 days and now it all has stopped and I have no blood at all. Is this normal and is this considered my period??