It is always your choice to decide when to test, and when to accept a test as conclusive. However, this forum has offered you all of the available information, so there is no need to continue to ask here about testing.
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it's not 5%, reading Dr. HHH posts its less than 1%, 99.8% or somewhere around there. close as you can be to 100%. people get headaches all the time, that doesn't mean anything. if you look for symptoms, you will find them. if you had no idea what the symptoms were and just happened to have a headache one day, i doubt you would be concerned about HIV. symptoms are never used to diagnose HIV only tests, which in your case was negative.
also, can you provide what your exposure was that youre concerned about?