Whatever the case may be, common reactions to chocolate in those who are affected include headache, heartburn, skin rashes, and breathing problems. Sensitivity to chocolate ingredients can also trigger asthma attacks.
One reason for the prevalence of reactions to chocolate is that a good part of mainstream chocolate products in the United States contain several additives; it's these that people react to. The purest forms of chocolate contain cocoa liquor, cocoa butter, sugar, maybe vanilla – and nothing else. Mass produced chocolate, on the other hand, is often primarily composed of the additives mentioned above (corn syrup, dyes, etc.).
If an allergist has diagnosed you with an actual cocoa allergy, of course you need to stay away altogether.