I get severe headache when i smell red onions. is this an allergic reaction?

No: This is an irritant reaction but does not represent an actual food allergy. Food allergy reactions are more likely to present with a combination of the following: rash or hives (the most common symptoms), swelling of lips, tongue, or throat, respiratory symptoms, vomiting, or diarrhea.

Maybe but: It may also be an intolerance. Either way the only treatment is avoidance. Sometimes cooking a food lessens such a reaction.