According to Freedom Health, P24 Antigen is highest in the Primary HIV stage when first infected, but it will stay at low levels throughout the course of the illness. So it maybe detectable throughout, but is decreased significantly. I am not aware of Colloid Selenium test. What generation was the test? If it was 4th Generation, then that tests for both antigen and antibody, while 3rd Gen tests only for antibody. You are way past the window period and would have detectable antibodies. I would suggest to test again with a 4th or 3rd Gen.
I already have indeterminate western blot result , which is forth generation test , just showing p24 reactive band