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.
Was it oral, anal or vaginal and did you use condom?