What Causes Irregular Menstrual Periods?

hi im 19 years old and always have a normal 28 day cycle but the last two months i havent! last month it was 37 days then this month it was late also i took a pregancy test but it waqs neg. then i got it it 40 days from my last about 12 days late! i dont no why nothing has changed in my life no stress or any thing i havent been on birth control since nov. 2009 ??