What Causes Delay In Menstruation?

hi i am a female i have not recieved my menstal period this month i usually get it on the 6th it is now the 26. i have taken 4 over the counter pregnancy test they all came out negative. just 2 days ago my fiance did ejacualte inside me multiple times so i was wondering if i was not pregnant when i miss my period and i became pregnant a couple days ago how long should i wait to find out and if there is just something wrong with my urine hormones??