why am I bleeding again?

I was two days late so I carried on taking my pill as I was going away for the weekend and didn't want my period. I then stopped taking It two days later to get my period. I still didn't come on for another three days. I took a test just in case I was pregnant and that's why I wasn't getting my period but it was negative. The next day I came on. I had my period for 3 days and then stopped. (I'm usually on for 5 days). I then went bleed free for a couple of days and now I've come on again.. Why?