What Causes The Periods To Reoccur Within Short Time?

hi, i have been off my contraceptive injection since june 2009 have had occasionally had a few periods since which i know is normal before they return to being normal and regular. i had a period in feburary and it was normal 5-6 days normal flow etc fr me. i do suffer from bad cramps. always have done. my last period started on 18th april and lasted til the 27th. then on 2nd may i started light bleeding and severe cramps the same on the 3rd and again today. i was just wondering if you can shed some light on why this might be happening?