I am experiencing light spotting/abnormal vaginal bleeding one week after menstrual period. i am on birth control. noticed after having intercourse.

Breakthrough bleed: Breakthrough bleeding (bleeding at the wrong time) is very normal on birth control. It happens in about 1/3 of users. Its no big deal and usually self corrects over time. If you are not satisfied or it continues to happen then consider using a different birth control method. It is a common side effect and does not mean something is wrong or that the pill is not working.

You could have some : A small polyp or cervical erosion (changes which can occur on OC) ,so that as the thrust of the penis making contact with the cervix , it causes bleeding