Were you exercising at the time of the allergic reaction? You may have exercise-induced anaphylaxis, which is an allergic reation that is brought on by exercise. I get a bad reaction to shellfish (hives, trouble breathing) if I ride too soon after eating it. I'm not allergic to it if I don't exert myself.

Not sure if that would make you feel exhausted. I'll recommend what my mother always did when I wasn't well--eat and sleep!