welcome to HCM
the brownish discharge is associated with many conditions among which some are most alarming. however it depends upon the time of the discharge. a continuous brownish discharge throughout the days may be due to cervical cancer
. or it may be due to sexually transmiitted disease which you have already ruled out by sending the swab for culture. now if the discharge have started with started without any menstruation
then it is most probably due to endometrial implantation
by the fertilized egg however if start just after the menstruation then it is the cleaning process of the vagina by shedding off the old endometrial tissue.
further test you can perform to rule out some condition is pap smear for cervical cancer, Beta HCG for pregnanccy and pelvic ultrasound
for pelvic inflammatory disease
. furthermore i would recommend you to switch to injectable contraceptives which is effective for about 3 months as the COCPs is associated with lots of serious adverse effects