What Causes Multiple Periods In A Month?

Hi, I have had three periods in the space of 4 weeks. My first period was late and very heavy and lasted around 12 days, when usually my periods last only about 5/7 days, it then stopped for a few days, in these few days I was having unprotected sex, then after a few days my period started again, this time very light and brown coloured, it stopped after about 2/3 days then it stopped again for a further few days and it has just started again, this time heavy, again. I have been having unprotected sex for the past 6 months and I am not using any form of contraception and I'm not on any medication. Please help.