Since its 5 months since you had the Implanon
replaced it seems a little unlikely that the bleeding is due to Implanon.
Bleeding after sex could be due to other causes such Cervical polyp
, cervical erosion, which needs to be checked by a Gynaecologist.
Could you please tell me when did you have your last PAP smear?
The bleeding could also be due to pelvic inflammatory disease
Kindly see a Gynaecologist and get an internal examination done for the same.
Hope this helps.