Missing periods


My periods have been all out of whack..

I have had months where they have been on time, then there are other months where I don't have one for 2-5 months.. people say it's stress.. and to be honest, I am not any more or less stressed then other months when I have been on time.. went to the doctor once and she said that everything seemed normal with a pap smear..

for example, this last time..

I missed last month's period. And this month I spotted for 3 weeks.. today I officially started..

Am I the only one?? What is causing this?