Was it oral, anal or vaginal and did you use condom?
While it is a risk, let's keep it in perspective. It is still a LOW risk with less than 2 percent chance with a single exposure. Why are you not believing your test? 3 months is conclusive. Why wouldn't you consider a test done at 25 weeks conclusive? Symptoms are never used to diagnose HIV, only tests and your test is negative, no hiv.