Confused and frustrated

I've had irregular periods in the past and thought they were gonna be regular. I got my period Aug 20 than sept. 18th ,Oct 25 and nov 24. And I have yet to have a period in December. I had sex nov. 27 (during my period) and my period stopped the next day. Than I had sex dec. 17th ( both times using the pullout method). I'm not sure if it's stress or not I took a test the other morning and it was negative. Any ideas?