What Causes Vaginal Spotting After The Periods?

hi , i am 31 yrs old . i am having spotting for last 15-16 days . My periods were regular then in july 2009 my period was 11 days late. then after every 3-4 months my periods are 11-14 days late . In sep 2010 i had spotting after my period . in dec my period was 12 days late but 3-4 days before that i had spotting again ... now i am having spotting which is dark brown and thin layers as well .. please tell me what this is ? i had my pap smear in july 2009 it was clear ...