where is Aunt Flo??

Okay so I had sex about two months ago. I know that I am not pregnant because I have had two periods in between the time that my boyfriend and I have had sex. But it's day 31 of my cycle and I still haven't started my period. That is very unusual for me. I think I may have ovulated later. Like 3 days after eve said that I would. Does anyone know what could be wrong with me? I'm freaking out!