What Causes Delay In Period Along With Tender Breasts And Stomach Cramps?

I had unprotected sex for the first time the 4th and the 6th of March . I was suppose to start my period the 13th of March but sometimes I m irregular. I am now 6 days late. I did have some spotting for 3 days a week ago yesterday. I have been having early pregnancy symptoms, tender breats, cramps, nausea, and I have been extremely tired lately. I have taken at home tests but they have come back negative. I m not sure if it s because it s too early to tell or what. Please help!