All of sudden irregular cycles?

Ka

I've been regular (28 days) on the day with both ovulation and my period ever since I started menstruating 6 years ago. I recently aka 3 months ago stopped taking the pill after 9 months. Again, the last two cycles after that have been regular. I'm on cycle day 20 today and only ever got a positive ovulation test today! Has this happened to anyone else? What could be a cause for this sudden change?