What's really going on?

I have been having sex with a really close friend of mine for the past two months now, I actually had my period July 7th and at the end of July I experienced some spotting of brown blood for 3 days than it went off. My period was supposed to come on August 10th but still hasn't come on yet, I actually went to the emergency room on August 13th & 16th for some issues and both times they did pregnancy tests through my urine and it came back negative. So what do you guys believe could be going on?