This would almost certainly not be "AIDS".
It takes several years ( on average 7-10) from the time of infection until someone's immune system is impaired enough that they develop the symptoms and infections we refer to as AIDS. Near the time of infection, patients often do get an initial retroviral syndrome, but that would not include "severe unexplained weight loss".
If the person was indeed negative foir HIV antibodies a few months before, they would not have "AIDS". Of course, they could still be HIV infected if they were in the "window" period when last tested, or if they became infected after being last tested. But that is a different thing than the question being asked.
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