missed period

My ovulation time was April 5th-April 11th, on April 16th i had unprotected sex. My period always comes on time like clockwork. My period was supposed to start April 23rd-April 28th but it hasn't. on April 28th I took a pregnancy test (clear blue)digital and non digital, and it was negative. Is it too early to tell, or could this be a medical problem,etc? today is the 29th and i still haven't gotten it, and i dont feel any period symptoms that i usually get before mine comes...should i try again? if so when?