What Causes Delayed And Short Monthly Period?

Okay I Got My Cycle Three Days Late And It Only Stayed For Four Days When My Normal Cycle Is 5 Days Long And Always On Time And I Dont Think This Could Be Pregnancy Because You Don't Have A Regular Period While Your Pregnant Or Do...And I Don't Think Its Stress Related Either {If This Helps I Did Have Unprotected Sex On The Following Days If This Helps Any One To Help Me May 3 May 7 & May 21 And My Period Was On April 14} Could I Still Have A Chance Of Being Pregnant