Not only have I smoked, but only two days prior I had had sex with my boyfriend. My period came on the twentieth, which was a Thursday, and normally it lasts for 5 to 6 days. The start has always been a rather heavy flow, so everything was normal, then on Friday, the 21st, I smoked with my boyfriend and his friends. The next day, on the 22nd, my flow just kept getting lighter and lighter throughout the day, until I'm almost positive it's at a stand still now. Is it more likely that I'm pregnant or just the smoking causing an irregularity?