Wife might me pregnant? Help?

Her period was supposed to starts on 10/25/2012 and it is already the 26th and she still hasn't had her period. She took a pregnancy test last night and it came up negative and she took one during her break at work today and it came up negative as well. We used first response early result test strips, they supposedly give you results six days before your missed period. They both are negative, do you think there is still a chance she might be pregnant? I am worried she is. Do you think I am over worrying and the test strips are right?