Could I be pregnant if period follows 11-day intercourse?

Its absolutely normal for periods to get delayed by few days or so. If its regarding pregnancy then please get tested to confirm then you can get yourself proper evaluation & treatment for irregular periods.

Hormone shifts, stress, and pre­gnancy can also cause this. Signs include a skipped pe­riod, feeling queasy, and sore­ breasts. To confirm pregnancy, take a te­st. Using protection helps avoid unwanted pre­gnancies and STIs too.

After unprotected intercourse, taking the emergency contraceptive pill (I-pill) can prevent pregnancy by blocking fertilization. Since you took it within 24 hours, it is more effective. Having sex on the last day of your period usually means a lower chance of pregnancy. Pregnancy symptoms typically appear after a few weeks, but missed periods or nausea can also be signs. If you're feeling anxious, consider taking a pregnancy test.

If a girl has sex before her expected period and then it comes on, she is likely not pregnant. However, some women may have light bleeding or spotting in the early weeks of pregnancy which can be mistaken for a period. If your girlfriend’s cycle was normal with the usual flow for 3-5 days then she is probably okay. Periods are sometimes irregular due to stress among other things so I wouldn’t worry too much about it unless there are other signs as well. 

If you ovulated on the 8th day of your cycle, you might consider taking a pregnancy test around 10-14 days after ovulation. Your expected period could be around the 22nd day of your cycle if it's 28 days long. If your period is late, taking a pregnancy test after missing it is recommended for accurate results