Hello. Thanks for writing to us at healthcaremagic A negative screening test at 12 weeks is conclusive that you are HIV negative and require no further testing. A screening test for HIV depends on production of antibodies against the virus; most of the infected individuals develop antibodies within a month and test positive. However, some infected individuals may not develop detectable antibodies at 1 month thus giving a falsely negative result by screening test. However, most of these infected individuals who were initially negative or indeterminate, become positive at 3 months. Therefore testing at 3 months is taken to be conclusive. Since your 12 week test is negative therefore one can safely assume that you did nt contract HIV from this particular sexual act. Most likely the gastrointestinal symptoms are related to some other unrelated cause. I would suggest that you consult an internal medicine specialist for the needful. Regards