Using a condom properly http://www.smart2ask.org/Contraception.htm#Condom
is critical but even then is not 100% guaranteed, though it's close. If "pre-cum" (pre-ejaculate) was present or semen got on someone's hands and then to your vagina there is some amount of greater risk of pregnancy.
However, none of that means you are likely pregnant. It mans there is a chance as there is *some* chance whenever you have sex. Your period could have been delayed through stress, if nothing else. Take the test and find out.
But you can take pregnancy tests like First Response up to five days before a suspected late period (aka 5 days before your period is supposed to come).