Generally if a person is infected with HIV the test will turn positive after three months. If your last potential exposure was December and the March test (~4 months post-exposure) was clear, it's overwhelmingly likely that you are not infected. In a very small percentage of cases the test takes 6 months to show up positive. If you're getting tested again in August that will be ~9 months later, well past the latest possible date for seroconversion (the point at which the HIV infection shows up on the antibody test) and if that test is negative than you are not infected with HIV.
You do not need to continue getting tested unless you receive another possible exposure. If in future you are at heightened risk (have sex with another man, or have multiple female partners) you should get tested every 6 months to 1 year, until exposure ceases and you test negative 6 months after the last exposure.
And of course, use condoms.