hi there, as you have missed your periods I would suggest that you take a pregnancy
test at the earliest and get an ultrasound
done as well. If you are positive then a transvaginal scan will be able to rule out any extra uterine pregancy which could be the cause of your lower abdominal pain
. If you are negative then you need not worry, take an antispasmodic and you may get your periods in a few days. Take care.