I have already explained the most common reasons why a false positive result occurs.
Most of us who respond on this site use more technical resources for information, but I have pasted a website for you that will hopefully give you an insight into how complicated your question can be to answer.
If you have further questions regarding false positive results, please direct them to an HIV Specialist.
http://www.aliveandwell.org/html/questioning/66reas.html
It takes time for seroconversion to occur. This is when your body begins to produce the antibodies an HIV test is looking for — anywhere from two weeks to six months after infection. It's possible that you seroconverted later than average.
It's also possible that a co-infection skewed the test.
What you really need to focus on is that they called you in for a CONFIRMATORY test which was negative. You do NOT have HIV.
I understand your anxiety and not wanting to go through the testing procedure again, but if that is what will give you, finally, peace of mind, then have another test. It seems to me this will be the ONLY way you'll be able to move on.
You have three options:
Accept your confirmatory negative results.
Test again.
Continue to live with fear and doubt.
I wish you the best
RubyWitch