Hi and thanks for the query, the commonest cause of a missed period or period disturbance or distortion in a female of reproductive age is a pregnancy. In your case, the timing of unprotected sex gives very limited physiologic chances of getting pregnant, but its possible for one cannot predict this 100%. However, Having taken the pill, which off course is hormone., has the ability of distorting your normal cycle flow. Its not unusual to observe such delays or at times early bleeds than expected when they are taken. I suggest you stay calm for now, for chances of pregnancy are pretty low but not completely excluded. You might redo another test, preferably a blood beta HCG level measurement in a week s time. Thanks and best regards, Luchuo, MD.