Hi there. Well, there could be different reasons. Are you severely underweight or work out to extremes? Some women don't have a period under those circumstances. You could have a thyroid or hormonal imbalance both of which separately or together could cause this. I personally had super long cycles, with a period every 3 to 4 months due to a slow thyroid (hypothyroid) and a hormonal imbalance (produced too much estrogen) which was going on at the same time.
Now, I'm hoping you are seeing a doctor. This is important. My doctor wants a woman to have a period every three months. Once it passes three months, you should be evaluated. Then they will either treat the underlying cause of the issue or do something to kick start it.
But I know it stinks. I'd get pms at the regular time as if I were going to have a 28 day cycle and it would last until I got my period 2 months or so later. Whew, what a huge pain that was!! So, I feel for you. Mine righted itself but I was seeing a doctor throughout the duration of that time. You should too. good luck