What Causes Delayed Periods?

hi...my name is sumaiya n i m 20yrz old...i had last intercourse on 19th april...n the due date of my period was on 24th april which started on 26th april nd ended on 1st may....but i dd not have any intercourse after 19th april...and i took 2 pregnancy tests on 3rd may and the results were negative...but i m missing a period now...am i pregnant???