Having irregular periods but no PCOs. Any idea?

Hello ladies,
I am neeraja. I really need your valuable suggestions.

My problem is i had an irregular periods and I went to see a doc and doc told that I am having PCOs in last year October 2013 and I took meditations for that. I got my periods regularly for 5 months. I got my last period is April 2014 after that until today I did not get my period. I went to see a doc on June 6th and doc did an scanning and told me that right now I don't have PCOs. The problem has been resolved.

He suggested me to wait for 2weeks and see if the period comes or not. I am not pregnant (confirmed through urine test and scanning). Again I went to see the do on 30th June because I did not get my period after April 2014.

on Monday he suggested me that I have to for a blood test and I did it now I am waiting for my results.

my main concerns is what could be the reason of having irregular periods other than PCOS. I am so worried is there any serious problem.

Other than PCOS problem what are the causes of irregular periods?
Please give your valuable suggestions so that next time when I go an see a doc I can ask him.