Welcome to HCM.
It is very difficult to rule out the possibility of delay in antibody formation in your body against HIV and thereby there could be a possibility of a false negative HIV test
in your case as you had Steroids
Anyhow it is a very short course and very unlikely to have such a false negative result in the presence of HIV infection in you.
It is better to have the test repeated after another 15 days to get another negative HIV test and to have extra confirmation that you are free from HIV without taking any steroids.