I can understand your concern
Which 28th day of the cycle are you talking about?
28th day of the missed period cycle or 28th day of the present cycle ? also when did you involve in unprotected sex ? I mean which day of the cycle? And are your periods of 28 days or more ? Are your periods regular ? You can take a urine pregnancy test on or after atleast seven days of your missed period.
Consult a gynaecologist in person for further information on this matter.
I hope I have answered your question well.
Dr . Anil Kumar Jain.