When is a menstrual cycle considered to be irregular?

I agree that seeing a gynaecologist directly might make the most sense as regardless of history taking by the GP, the ultimate confirmatory diagnosis is still ultrasound of your ovaries. They would be better placed to discuss with you on treatment modalities.

I suggest to have a scan done by your gynae to check the lining of the endometrium. It is not uncommon after prolonged use that the lining thins out by quite a bit so there is nothing much to shed as a “period” as such after cessation of the pills. This situation is generally temporary, and the periods should come back soon. A second possibility is that you naturally had infrequent cycles (say every 3-4 months) due to irregular ovulation. The pill will mask this and make it seem like your periods come every month.