What Causes Menstruation Twice A Month?

I'm 28 yrs old and single. I've been having some irregularities in my periods, I have a 7 day cycle, but this time my period ended by the 4th day, though I still have some pain in my breasts and feel I will have them again after 15 days (my cycle is 26-28 days). I have had periods twice in one month a couple of times before too.
Should I be concerned?