Have brownish discharge after sex. Did Pap smear, hormone and STD tests all normal. Suggestions?

I m 28 female Asian. I finished my period on 3/1/13 today is 3/10/13 . I have 14 days left to receive my mext period. I had intercourse on 3/8/13, 24hr after intercourse I experience brownish bleeding, the normal stretchy mucus like discharge is now dark brown. I had this issue started in February 2013, I went to the gynecologist in Feb, did a Pap smear , did hormone testing, did all STD testing and were all normal. Doctor advised me to monitor in March and see if it would come back and now its March and I m experiencing the same thing (2nd day of brownish bleeding). In Feb, the brownish discharge lasted for 10 days. I finished my period on 2/1/13 and started experiencing the brownish bleeding from Feb 9th thru the 16th. doctor said even the hormone blood test was normal, if this bleeding comes back, he would advise me to take birth control pills. Now my question is, since the Pap smear removed the possibility of cervical cancer . Can the brownish bleeding come from the uterus or the ovaries. I also did an ultra sound of my uterus, ovaries and cervix in August 2012, they were all normal. The brownish bleeding is bothering me a lot and i dont know when this bleeding will stop in March. hopefully someone can give me some guidance on what else I can do to find out the real reason for bleeding. Can this bleeding really related to hormonal changes? Can it be my uterus? Please advise.