What is this?

Had my last period from May 23-28, it was very light the whole time which is unusual for me. I had sex the 30th and had to take a plan b and I know that can fuck up my ovulation cycle but could it make my period come this early?

Also I had thought I was pregnant for a while because of sex that I had on May 15th, but then I got my period, and also took a pregnancy test last night (which came out negative) just to make sure.

So any ideas?