What Could Cause Brown Spotting Before Periods After Having Sex?

Hi Dr. Around the first week of June my boyfriend and I had sex quiet often and a week or so later I started getting bad back pain and started feeling dizzy. Then around June 17th I started spotting some brown discharge but only noticed when I wiped which last 5 days. I took 2 pregnancy test and both came back neg. Then after those 5 days of spotting I started my period. This month has felt different though. I can normally tell when I am going to start and this month It came out of nowhere and I have had no cramping at all which is unusual. Any idea what's going on? Why did I have spotting 5 days before my period??? Thanks for your help!