Hi, and thanks for your medical question on eHealth forum!
If you had unprotected sex and the periods came as expected the next day, then the pregnancy test is accurate. You are not pregnant.
However if after sex you had vaginal bleeding which did not correspond to your periods and now you have cramps and tender breast, then you need to rule out STD, infection, PCOS and pregnancy. Then the ideal way to do is to take the test on the due date.
Since a confirmed diagnosis cannot be given on net please discuss these possibilities with your gynecologist once. Hope this helps. Take care!