I would definitely go see a doctor
about this issue. Making an appointment with either your primary care doctor
or your OB GYN doctor would be appropriate.
I agree that it does seem unlikely that you are pregnant, given the fact that you had a relatively normal period about 2 weeks after having unprotected sex. However, anytime a period is late, as is your case, the first step is to make absolutely sure that this is not a pregnancy. Your doctor can perform a quick pregnancy test on a urine sample to rule this out and, if it is negative, they can then proceed to evaluate other causes of your delayed period.
Another possibility is that you did get pregnant after having sex but that the pregnancy was not viable and that the bleeding
you had two weeks later represented an early miscarriage. In this case, after a miscarriage, it is often common to have some menstrual irregularity for a while as your body and your ovaries "reset" themselves.
Furthermore, your doctor will want to know if you have any prior history of irregular periods, or if this is a first time for you, as there are several conditions that are associated with irregular periods as a more persistent symptom.