What Causes Delayed Period While UPT Is Negative?

okay last time i got my period was july 26.. but i have an irregular period. im supposed to get it every 11th. but i had sex for the first time like august 7th, and i STILL dont have my period. i went like a week an a half ago to the doctor to get a pregnancy test and i was negative. two days ago i did another pregnancy test and its negative.. what do you think?