That's curious.... Allergic reactions are when the body's immune system detect something that it recognizes as a foreign substance (ingested, contact through skin, food or medicine, or in the case of auto-immure diseases, it's own cells). One reax is to mobilize cells that secrete anti-histamines to fight the allergic reax. So, allergic relax to antihistamine would be very unusual. Let detective doc sort this out.
Red herring? It would be exceedingly unusual to have a true allergic reaction to an antihistamine. You'd need a blinded, placebo-controlled challenge to establish a true causal link.
Exceptionally rare. Probably never happens; almost alway the antihistamine in question is either inadequate to control the allergy (such as hives, swelling, leading to blood pressure drop) or another cause is found. I would seriously contact an allergist who can methodically evaluation your situation.