What Cause Two Periods In A Month?

hi there my name is leah im 20 and i have been on sereval different contreceptives the pill , the mini pill and the depo injection none of them agreed with me so i 2years later i was given cerazetter i have been on cerazette for a year and decided to get back to a normal cycle and stop taking it (mid way through packet) january 2011 i did not have a period then i come on 4th of march for three days. 2weeks later i come on again (today) and im heavy im so worried something is wrong with me. i keep reading things online. please help me i cant sleep my weight is normal and im healthy and feel fine. please please help me work out why i have had 2 periods in one month thank you