I Am 26 Year Old Female. My Fiancé And I

I am 26 year old female. My fiancé and I had sex on September 6 on that exact day I was ovulating. Yesterday September 17 I woke not feeling myself, I was cramping feeling like I was getting sick, and my stomach was upside down going to the bathroom all morning, afternoon and evening. I started to spot last night but today my period is super watery and very red. Why is that? I did take a pregnancy test yesterday morning but came out negative