Sounds like you had a real period after sex so you can not be pregnant. The pregnancy test confirmed this.

Please be more careful next time. Better to have condoms on hand for occasions like this than to be wondering and sorry later.

It was probably too early for you to have taken a pregnancy test; and most research suggest that you wait until the period has been missed to take a pregancy test. A false negative test can occur for several reasons, and one of those reasons being it was too early for the test to detect a certain hormone present in high forms in pregnant women. In some cases spotting or bleeding can be misconstrued as a period when it is simply a process called implantation. For more accurate results you should see your physician, at which point a blood test or urine analysis test can be done. You, like most women, should understand that you can get pregnant at any time despite times where conception is very unlikely. Most research suggest that conception is very high right before getting a period, lower in the beginning of a period, and higher towards the end of a period; and this is mostly because most women don't experience a regular menstrual cycle, which makes predicting ovulation difficult. In addition, you stated you had some clotting during your period which could suggest a miscarriage or some other serious problems. You should consult your primary care physician for a more accuarte explanation. Good luck.