Urine tests are extremely accurate; otherwise they wouldn't be on the shelves *sighs*
Yes, you could still be pregnant. If there is some way to get an ultrasound or other test done, I'd do it. A blood test may help, but be sure to tell them that last time urine tests weren't accurate until 4 months in. That way they can use the most accurate blood test available.
The usual blood test used is actually LESS accurate than the average urine test. Why? Because by the time the woman gets a positive urine test and goes in for the blood test, there is enough hCG in her system for an insensitive test to detect.