Basically, since the end of 2011 my periods have been off whack. I kept coming on at the very end of each month, I came on new years eve, then january 29th then february 28th etc.... but I went to cyprus in july and figured Id be okay because my period would be sometime at the end of july. However two days before I went away I came on (early) on the 16th july, but came off on the 19th. The shortest period I've ever had. I had cramps and all. 
Then I came back from holiday on the 25th july, and on the 28th july I came back on my period, this time the normal amount of time, until the 4th august. I figured being abroad may have messed things up half way through my period and it had re started but now I've come back on again, 8 days after my full period I am bleeding again and having cramps. It can't have been the holiday because I came on early before I even went away. Does anyone know what this could be?
Im 21, Im on the pill, and I never have unprotected sex with my partner. I have always had normal periods.