Some women don't get a positive until after AF is due by quite a few days. Personally, if I had a negative but AF still hadn't come after being a week or more overdue, I'd call the doctor. Very few women don't produce positives with urine tests and a blood test or internal u/s would be required to verify a pregnancy. If you weren't pregnant, they might be willing to give you a Rx for something that would kick start your AF.
Our bodies can be mysteries!
Rae (27), DH (26), Lucas (6/3/12), #2 due in December