Worried. My period cycle is every 3 weeks, last time- 2 weeks, now its been over 3 weeks & not yet. Pregnant?

My menstrual cycle is every three weeks, a little shorter than most women I know. During my last cycle, however, my period came after only 2 weeks which I know is not normal at all. I lost my virginity 2 weeks after this period, which would be 6-7 days before my next period if I was to return to my normal 3 week cycle and I'm now a day late (again, uming a return to the 3 week cycle.) What I want to know is 1. Am I correct in uming a return to the 3 week cycle after the 2 week abnormality? and 2. How many days past whenever this expected time of my next period is should I be concerned about pregnancy?