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.

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 

Taking Unwanted 72 after unprotected sex can help in case of pregnancy. It might have to be mentioned that young people should take it within 72 hours. Since the usual menstruation time has passed and in the meantime, you've already got your period, the symptoms you may develop are mostly related to the bleeding. Spotting or changes in the menstrual cycle are the main issues after taking the pill.