Protected sex might lead to pregnancy
especially if there was any condom rupture during the act.
Now that your period is delayed beyond 2 and half weeks of due date you first have to get a urine test for pregnancy done. It is a simple test which can be done at home using commercially available urine pregnancy test kits . You can get it from any chemist shop.The instructions, results and interpretations are mentioned in the kit itself for self diagnosis.
Now if your test comes negative then you might need to wait for a week and repeat. You also need to get a pelvic ultrasound
done to look into other gynaecological causes of irregular menstruation
In case the test comes positive then also you have get a pelvic ultrasound done and see a Obstetrics and Gynaecology specialist regarding further management.
So to summarise investigations and suggestions are:
1. Urine test for pregnancy.
2. Pelvic ultrasound.
See an Obs and Gynae Specialist at the earliest.