Irregular periods

Brittany

Hi. I am a bit confused with my menstrual cycle. Am not sure why this happening, but I normally have a 28 day cycle and since 2020 has begun it has been very irregular. Sometimes its 21 days, 28, 30, 36. The last time I went to the doctor he said it can happen I should not be worried, but now 2020 has ended I have not seen any period for December, and I am not pregnant. Should I be worried??