I haven't had my period in 3 days?

this can be for many reasons number one stress can affect the oncoming of a period if there is no stress it can just be late thats what periods do they are known pretty much can be on time for a while then all of a sudden for no reason can be late its a known happening with periods unless your sexually active birthcontrol can cause it also medical virus can cause it also the flu or other medical issue a common cold could cause it to be late