What Causes Delay In Periods?

Hi. I am 19 years old and my period is usually normal and I never miss a month, except now. But I didnt have sex with a guy in over two yrs so I know that it was related to stress. So I should have gotten my period around the 20th, and didnt.than on the 27th i had sex with my bestfriens and the condom broke and he came in me. Now in my past I have been pregnant twice before and I am very fertile, and he has gotten his ec pregnant 2 but none of the pregnancies went full term. And now im am so scared to be pregnant but I think there is a very strong possiblity. Can you please help me out and tell me what is probably going on in my body?