What Causes Prolonged Periods And Abdominal Pain?

I usually have my period every 28 days like clockwork. Well two months ago i had my period in the begining of the month and again at the end. my period always lasts about 4 days. day 1 being heavy then 2 3 and 4 being light and brownish. well last month i skipped my period all together and on the 1st i got my period this month. Well i used to get it for 4 days but its going on day 9. its very light and very bright red. sometimes when i stretch my lower abdomin hurts. i am not on birth control. and am 21 years old