HELP!!!!dont wanna get prego but I suspect Im one week on the way what should I do?

got my period last 18th of october ,I have a more or less 35 cycle, my next period would be on the 22nd of november, but 2 days ago,I got the what they call pre-implantation stain.Is it possible that I am not pregnant and the stain is just a prelude to my period this week???I feel bloated and my stomach is crampy,I had a loose stool 3 days ago but now I am constipated.Got pain on my too.