Is A Delayed Period Signs Of Pregnancy?

am 42 yrs old... long time no sex ....lmp july 5 2011...... then had unprotected sex last aug.7.. i thought my peroid will come after had sex.... am expecting my period last .. aug. 5... but now aug.15 ... still missed period .. after had unprotected sex that day, i take pills immidiatelly 18 hrs after unprotected sx . till now taking contniously the pills .why still i dont have my period? i take P.T .. but negative last .. Aug. 13..