What Could Cause Delay In Periods?

my period is EVERY 27 days and mine is about a week late I do not have good luck w/ home pregnancy tests due to the fact that 3 times before they al came back negative and I was in fact 2 months pregnant each time grrrrr. my period is about 5 days late so I called my OBGYN who did a blood test and that too came back negative, yet I suspect I may be pregnant is this possible?