Why is it recommended to go back and retest for hiv after three months?

If you tested negative to HIV within the first weeks or month you would retake the test because the body develops different antibodies that help 'fight' off the virus. Therefor your body would be in the process of trying to get the virus out of your system in which most antibodies and symptoms would start to show itself. There are specific test that are made to spot and identify these particular antibodies that tell you whether you have the virus or not.