I'm wondering if this is normal?

I had unprotected sex at the beginning of June, during or a day after ovulation. I'm usually on a 28 to 29 day cycle so when I was scheduled to start on June 21, nothing happened. On occasion, my period has been a couple of days late but I never worried about it before. Anyway, a week passed and aside from the cramps I'm used to having at my time of the month, there were no signs of my period coming. I took a pregnancy test and it came out negative although one line was faint. I wasn't sure if that was a false negative, so I took another test of a different brand and that also came out negative. This morning I woke up and had very intense menstrual-like cramps except that they were about ten times worse. The only time I had ever experienced pain like that was when I was in labour with my daughter. I ended up bleeding but it was brown and spotty, not at all like my periods normally are. Is there a possibility that the pregnancy tests were wrong? Or if I am not pregnant, what are some reasons as to why my period was so late? Please note that I am not under any stress whatsoever so I would not count that as a contributing factor. If anyone can shed some light, it would be greatly appreciated. Thank you.