Hi,
Welcome to ehealthforum.
The changed bleeding pattern could be due to pregnancy if you are not using any protection. The heavy bleeding after intercourse while being on periods could be due to more bleeding or miscarriage induced by the intercourse. Wait and watch how the bleeding is. If it is heavy, associated with cramps and clots, then do not delay your visit to your doctor/gynecologist for examination and tests to confirm the cause for it to start you on appropriate treatment as needed to stop the bleeding.
Take care.