Though it is not commonly labeled an allergy symptom, it is possible to experience nausea with allergy flare-ups. After prolonged exposure to an allergen, your body may have produced excess mucus buildup. As your body attempts to clear away this substance, mucus will begin to drain away as post-nasal drip. As it drains down your throat, mucus will begin to build in the stomach, inducing nausea and sometimes vomiting. Call WellCare Urgent Care if this symptom arises.