What Causes Irregular Periods?

Hi my name is jessica im 17 years old and im in need of anserws . ive been waiting on my period and i think it has been late even though my periods are irregular . my last period i think was 3/20/11. starting 4/28 i started to cramp it was odd because i barely even noticed it . then when i looked to check there was an brown discharge nothing else i decided to wait . then the next day it was the same except a little heavier clots and all. then the next day later on in the day i looked n there was blood all over my underwear like a brownish/red maybe and then brown again. and then yesterday the same thing happen it was more blood maybe reddish/brown . My stomach also has been feeling funny like kinda bloated especially in the morning and when i eat i get mild nausea . what is this ??