Thank you for sharing your question with the EmpowHER community.
It depends on what kind of test you had. There's a period of time after a person is infected during which they won't test positive. This is called the “hiv window period.” The window period can be from 9 days to 3-6 months, depending on the person's body and on the HIV-test that's used.
Antibody tests ("Rapid" tests) — give a positive result based on antibodies to HIV, not the virus itself.
Antigen tests (RNA tests) — show a positive result based on the presence of the virus. These tests are more expensive than anitbody tests, so are not offered in as many places.
Home testing kits — As of Fall 2012, there are two "home tests" which have been approved by the FDA for use in the U.S.:
PCR tests (Polymerase chain reaction tests) — also test for the actual virus. This type of test is often used for testing the viral load of HIV-positive people, as well as testing babies born to HIV-positive mothers. You can read more about PCR tests on the AIDS.gov website.
I hope this helps.