The window period or incubation period is the length of time after infection that it takes for HIV to multiply enough for a normal antibody test to detect it. Each person's body responds to HIV a little differently so the window period varies slightly from person to person. It usually takes up to about 3months after infection for HIV to be detected by a test. So have another test 3months after your 1st one to be sure. This 2nd test will tell your accurate result, as long as you were not exposed to HIV again in the time between the 1st and 2nd tests.
Attribution: Marie Stopes