If you had sex esp unprotected then yes it is a possibility. Did you take a test? If not I would do that first. Use your first morning urine for the most accurate results. Also if it comes up negative call your doctor as he may want to do some blood work. There are some things that can cause your period to be late like stress, exercise, or a weight loss/weight gain. So if you have any of these it could be affecting your cycle as well. Good luck and I hope you get your answer soon.
the only way you will know for sure is to take a HPT. And it that comes negitive I would wait a week, to see if you AF comes, if not go to the docs to find out why you are late. Good Luck