Then I had spotting - maybe implantation bleeding on day 8&9 post (possible) ovulation. 

Then day 10 po nothing, late in day 11 late in day heavy bleeding and the worst cramps I've ever had in my life. It was so unbearable that I started to question whether it was more along the lines of miscarriage than period. Next day same, the pain has been quite significant, with nausea and dizziness. Passing tissue with heavy bleeding. All in all this is nothing like my normal period. I think I probably conceived but resulted in a non-viable situation but I am very new to this. Any thoughts?