The half life of IVIG antibodies is roughly 3 weeks. By now, most would be out of your system. Two things come to mind - First, has your doctor reviewed your symptoms to make sure this isn't some sort of meningitis or encephalitic response?
Second, if the IVIG made your immune system stronger and you have a chronic infection (e.g. Lyme) then you could be experiencing a continued response to your body finally being able to attack the infection. When Lyme bacteria are killed, they release toxins as a last-ditch attempt at chemical warfare. This is known as a die-off, or herxheimer reaction (called a herx in the lyme community). When my son had IVIG, he experienced a 10-week long herx that was just horrible in so many ways. Because his body didn't "follow the script" and respond the way it should have for Pandas, we tested for Lyme through Igenex labs (waited 10 weeks to make sure all donor antibodies were out of the picture). His Lyme test was negative prior to IVIG but positive afterward, because the IVIG strengthened his immune system to a point where it could finally start fighting the Lyme and producing its own antibodies - which resulted in the test finally turning positive for Lyme.