Missed Period Negative Results?

My period is 4 days late now and i took a pregnancy test the day i was supposed to get it and it came up negative. I have never been late in over 6 years it has always been on time. I am going to retest this friday. Is it possible i am pregnant and getting a negative response from the pregnancy test (took first response 5 days before missed period test)? Has this happened to anyone else? When could i expect positive results if i am?