The HIV test is extremely accurate. The only time the test is ran twice, is when a result is positive, to rule out false positives. The only way you could've contracted HIV is if she was positive, not if you were wearing protection or not. Also, HIV shows up within the first 72 hours.

Actually the standard way to test for HIV is to test for antibodies that develop in your blood as a response to the HIV virus. It can take several months for these antibodies to reach levels that are detectable. You should at least wait 3 months after a potential exposure before testing and then test again in 6 months to a year. It is very rare that it would take more than 6 months for the antibodies to show up in your blood. I wouldn't worry too much because in the heterosexual, non IV drug using population in the US HIV prevalence is low. Non the less test again after about a year from the last time you had sex with her.

Here is a website that seems pretty accurate. I didn't read much but what I did read seems to correlate with what I know about laboratory testing.

http://www.avert.org/testing.htm