I would not call the period irregular if it has appeared late by few hours. The normal variation of bleeding pattern is usually what you have experienced. It should not be labelled as irregular or heavy. Little variation in the flow pattern may take place when you change the birth control pill brand. This is due to the hormonal
composition of the pills.
Unprotected sex a week before the expected menses should not cause any abnormality or variation in the menstrual pattern unless you have caught some infection. Infection would have caused painful heavy menstruation
and foul smelling discharge which you are not experiencing. So possibility of the infection also may be ruled out.
I feel you should not have any concern about this little variation in your menses this time. Probably same pattern may continue henceforth till you are on Yasmin
I hope this has satisfied you.
Dr. Nishikant Shrotri