Sounds like your estrogen is low. Have your doc check your hormone levels. You might have a cyst or something else going on.
I had really long cycles for over a year in which I had periods every 3 to 4 months BUT, lucky me, got pms symptoms starting at three weeks in and they lasted the whole time. What a joy, right? What my issue was included a slow thyroid which they call hypothyroidism and also a hormone imbalance in which I actually produced TOO MUCH estrogen. Those combined were deadly to my cycle. For me, they self regulated out of nowhere. I was on my way to an endocrinologist as in waiting for my appointment as it took a while to get one and thankfully it all resolved on it's own. I have been regular ever since. good luck