What Causes Missing Periods?

Am 24yrs, 5ft 5, 60kg. My cycle lenght is 35days more or less. Without having previous sex for 6months my period was delayed for 4days but on the 6th day i had unprotected sex and used postinor2 on the 8th day(within 72hrs). Now this is the 11th day am late and i have not seen my period.pls could i be pregnant or sick