An undetected NAT was a great indicator,
PCR tests directly look for the virus in the body unlike the Antibody test which looks for the body's response to the infection. Though it is not a stand alone test, it has to be backed up with an antibody test.
RNA PCR can detect the virus as early as 72 hrs after the infection and the detection just keeps getting better along with time since the HIV virus replicates itself in huge numbers every day, which means viral load keeps increasing every single day after the infection.
Not detectable only confirms that you didn't acquire the virus. Just get a standard antibody test at 6 weeks or a IV gen HIV test at 28 days to back this up. I promise your result won't change. You're okay.
It could, though it's not likely given the sensitivity of modern tests. Just test again with a 4th gen in a few days and call it quits. You'll be for sure conclusive by then.