What Could Cause Delayed Periods?

My girlfriend's period ended on 4/8/11 and we had unprotected sex on 4/10/11. Within 11-12 hours, she took Plan-B One Step. Looking at the box from the Plan-B, it says it stops ovulation. Seeing how she took it before her expected ovulation, i assume that is what happend. However, she is a little over a week late in her period and I was wondering if, assuming the Plan-B did stop ovulation, that could be the reason that she has not had it yet.