Brief Answer:
Possible cervical erosions/ dysplasia

Detailed Answer:
Hello,
Thanks for the query to hcm,
Any form of post coital or oral sex bleed in form of dark brownish spotting should be investigated and its most likely due to cervical erosions or dysplasia. it is suggested that you should seek a per speculum examination by a gynecologist and get a PAP SMEAR + HPV DNA contesting done to look for dysplasia of cervical epithelium or any HPV infection which can cause such episodes overtime after intercourse.
Regards

---

Brief Answer:
answered

Detailed Answer:
There is a third possibility of cervicitis or cervical epithelial inflammation which makes the cervical mucousa susceptible to frictional injury during intercourse. So this shall need an antibiotic course for a week to 14 days depending on the response.
HPV infection and pap smear has a grading to be followed , unless there are LSIL or CIN2-3 lesions, cervical cauterisation is not offered and treatment is follow up 6 monthly as HPV infection is usually self limiting.
As your pap smear has been negative, the likelihood of vaginal mucous abrasions due to frictional injury and potential vaginal dryness during the act can also be a cause and if confirmed the treatment remains conservative with self healing.
Thirdly , if the spotting is experienced even randomly during the cycle , especially in mid cycle or early secretory phase ( second half of cycle) then ovulation bleed and possible progesterone deficiency can be other causes.
So in that case a hormonal profile would be mandatory. If your fear is with regards to any cervical cancer then it is unlikely at your age and also as pap smears are negative so you are safe.
There has to be a local cause and that can be determined with a gynaecological examination. So please schedule a visit to your gynecologist.
Regards

---