What Causes Irregular And Short Periods?

I have been having irregular periods and last month i missed it altogether. I am normally very regular with a 28 day cycle and a 5-7 day period heavy flow. recently, past 4 months, my period have been coming later and has been shorter and lighter. I am sexually active so i took a pregnancy test. It was negative. Could i actually be pregnant and it didnt show up? or what else could be causing these problems