It may be the case that your reaction had nothing to do with you having been on oral steroids. Anyone can develop an allergy to something at any point in their life and it may be nothing more than a coincidence in time.
I'm not saying for certain either that the symptoms you have experienced are the result of an allergy but it sounds like it is.
Whether an after effect of medication or new issue/allergy like Marydoll advises if I was you I wouldn't eat any dark chocolat again.
July 23 Limerick (continuation of July 21)