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If you had periods on 14th Sept and then sex on 17th Sept, then also you cannot be pregnant as this is a safe time to have sex. As extra precaution you took an emergency contraceptive pill too. Hence you cannot be pregnant.
The delay in periods is probably due to hormonal effect of the emergency contraceptive pill. Just wait for some more time for the periods to come. If they do not, then consult your gynecologist. Hope this helps. Take care!