linenup
Could be a sensitivity to corn since most Vit C is sourced from this. Might want to check NutriBiotic's brand.
Some Vit C contains bioflavanoids which could be causing problems.
Could be a detoxification issue since Vit C will push out toxins, thereby raising BP.
Might be antagonizing (or causing loss) of minerals such as potassium and magnesium which are both vital for BP (keeeping it low).