Pregnant ?

My last period was August 14. My periods last 5 days. I was suppose to get my next period September 15, that night after sex i had blood when i wiped so i thought that was the start of my period; i put a tampon in and went to bed, when i woke up the tampon had nothing on it and i havent had any bleeding sense. Ive taken 4 pregnancy test the last couple days and even today and got the same results ((negative)) on them all. Was that little amount of blood when i wiped my period? Confused!!?? What should I do??