Oh, goodness. Pregnancy tests are usually administered first thing in the morning when your urine is most concentrated, thus HCG levels are easier to detect.
I took a home pregnancy test with my first pregnancy (my second was done randomly at the family practitioner's office), and then called my OB/GYN. They said their tests in the office were the same that you'd purchase over the counter, so if it was saying I was pregnant, I was.
It could be a bad batch (lot) of tests at your physician's office, it could be that it wasn't your first morning urine, etc.
Only the blood work will tell if you're pregnant. My sister had 6 negative tests (including those done at the OB's office) before a positive confirmation resulting in my 13 year-old niece.
Good luck!