Thank you for your query regarding irregular menstrual cycles.
It would be helpful if you could give me an idea regarding your previous menstrual cycles prior to this and history of irregular bleeds prior to this event.
In a women with a regular 28 day cycle the chance of conception within the first week is minimal, even more so if emergency contraception is taken within 72 hours.
Sometimes, some women may experience a withdrawal bleeding after taking exogenous progestrogens.
I would suggest a urine pregnancy test, followed by maintaining a menstrual diary for the next few months and consideration of a regular contraceptive agent.
Do let me know if there are any further queries.