Thank you for your query at DoctorSpring.com
I understand your concern.
Whenever there is a delay in period apart from medical causes like PCOS or hormonal imbalance, the other factors that are contributory are stress, weight changes, and anxiety. In fact stress can delay periods even by a week.
Protected sex rules out chances of pregnancy, and hence it seems to be due to stress factor.
If it persists even next month, and the delay keeps increasing, without the stress factor, further investigations will be required to localise the cause.
Let me know if i can help you further,