What Causes Delayed Periods With Stomach Cramps?

i had unprotected sex 4 days before my period was due. i am now 4 days late and have been getting period like cramps for the past 5days. i normally get a slight cramp a week before my period is due but it only ever lasts for 1day. what would be causing me to have cramps for this long without any other of my usual period signs? i did do a home pregnancy test which was negative.