What Causes Post-coital Bleeding And Pink Discharge 10 Days Before Periods?

Dear Dr.,
For the last 11 months I have been experiencing post-coital bleeding, however with a particular pattern – only 7-10 days before my period would start. The closer I would get to my menstruation, the stronger the post-coital bleeding would be.
I have done a pap smear 10 months ago and the result was Pap 1. I have also been tested for sexual transmitted diseases and Gonoc./Chlamidia infections. They were all negative, 10 months ago and this month, too.
Recently (last month and this month) I have also experienced a light pink vaginal discharge starting about 10 days before my normal period would start (again the same pattern as post-coital bleeding). This light discharge persisted for the remaining days of the cycle and got more brownish as I approached the first menstruation day.
Due to this new symptom, I have been sent for an intravaginal echography. Everything seems to be normal and no reason for my symptoms could be found. Currently I am scheduled for a colposcopy.
My cycle is regular, with normal bleeding. Just during the first day of my period I have severe cramps that go way after 1-3 pills ibuprofen (250mg).
Your thoughts on this would be greatly appreciated.
Thank you!