What Causes Periods Twice In A Month?

I am 25 yrs old and I get my regular period cycle around 12th of the month. but last month which is April,2011, I got my periods on 12th April and lasted till 7 days, which is normal as I get my periods for 7 days every month. But last month I got my periods again on 29th of april and are still going on and today is 11th of May,2011. Wat do i do. Is it serious. Please let me me wat is wrong with me and wat do i do for it.