Here, you should undergo one blood test
for beta-HCG & TVS scan (2-3 weeks after unprotected sex) to rule out possibility of pregnancy
In exclusion of pregnancy, causes of skipping period are ovarian cyst
(most common), hormonal
imbalance, over wt/ loss, irregular diet habit, mental stress, psychological factors etc.
Following above test report, you need to consult with gynecologist for other relevant investigations to pin point the diagnosis. Take suggestive measures accordingly.
Practice safe sex in future. Good luck.