What causes an adverse reaction to epinephrine? When given, my body did the opposite... heart rate slowed tremendously, almost passed out, bp very high, and immense headache. Emergent situation why did my body respond this way?

Paradoxical. When given intravenously adrenaline may raise the blood pressure so quickly that a reflex slowing of the heart rate kicks in. If you didn't receive it IV, the subcutaneous administration in your case may have entered the bloodstream so fast that a similar reflex slowing of the heart caused by rapid rise in blood pressure occurred.