My last period was step.14 and i had unprotected sex oct.13 and i haven't got my period what could this mean?

Several things: If Oct.13 was the only time you had sex pregnancy is less likely to be the cause but it is still possible so I'd recommend starting with a pregnancy test. If it is negative the delayed period could be due to stress, a change in your diet,exercise,or weight, medications, or diseases such as thyroid disease, insulin resistance, or diabetes. See your doctor for evaluation.Good luck.