The problem with determining what caused your reaction is that the cake contained other things besides sugar such as flour (wheat), eggs and probably food colorings if it was a mix.
You may have had a sensitivity to one of these things all along, but never reacted like this because you at them frequently and your body was used to it. Now that you've been off it for a while, the reaction is much stronger. My DH has allergies and has noticed the same effect with things that he was eating and now knows that he is allergic to (like nuts) but had been eating. Now, even a small amount sets off an immediate and powerful reaction.