Your presenting feature is suggestive of post coital bleeding. The most common reason is ruptue of hymen. Mild bleeding and pain are quite common. It heals automatically within 2-3 days without any intervention.
Other reasons are vaginal infection
, cervicitis/ vaginitis
, cervical polyp
etc. Here, you need to consult with gynecologist for pelvic examination and relevant investigation to rule out underlying pathology.
Hope this information helps you. Be well and practice safe sex.