What Causes Intermenstrual Bleeding?

hi, I am 28years old (Female, single). I had my period between feb 9, 2014 till feb 13, 2014. After 11days of my period i hav spotted bleeding in browish/black color for 4days( feb 24-feb 27 with very very little amount) and after it was stopped. Again the bleeding started 5days back with very very little amount discharged. Could you please let me know if this is serious problem. Recently I did my blood test that is before my last month period(Feb 6th) and reports were normal. My next periods would be on march 9. Please let me know if this is serious problem