What Causes Blood Spotting After Periods?

My last pdroid was dec 27 2010. It usally is 30 day cycle. On jan 9 and 10 I noticed a little drop of blood in the tissue paper...what is it. I haven't had vagnal sex in over a year but do have oral or dry sometimes...im not on birth controll but am very scared please give me any help...im not ready to have a child