since you have missed your period for the first time and history of unprotected sex you might have gotten pregnant.to check u can go to the nearest med store and get a pregnancy
kit and do the needful.
other causes could be stress
,any form of infection,or it is sometimes considered normal if you miss a period.
with age the amount of blood lost decreases so there is nothing to worry
and about your tightening in tummy it could be a spasm if it gets severe you can consult a near by doctor for an anti-spasmodic tab.