Spotting help!!!

I had sex three days ago and for the last two days I have been very lightly spotting and my period isn't due until the end of the month. Any idea what this could mean? This has never happened to me before. Also two months ago I thought I was pregnant and the time my period was due I got my period and I experienced the worst pain I've ever experienced through this period... It was worse than labor (I've had two children). Any ideas or thoughts would help PLEASE!!!