The birth control should have been started from day 1 of the fresh cycle and then the levels of progesterones and hormones can be ensured through out cycle , thereby preventing ovulation and hence pregnancy. If you have experienced post coital bleeding then it is likely that you have cervical erosions and which may cause bleeding due to frictional injury. You should visit your physician for a pap smear evaluation to rule out any dysplasia.
I hope i have answered your query in detail,
wishing you good health,
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