It could look like any other allergic reaction, of which symptoms may include: Hives and generalized itching Nausea, vomiting Diarrhea Abdominal pain or cramping Skin redness Nasal congestion Cough Many people on this site have also said that their lips swell up if they eat fish they are allergic to.   You should also be aware of anaphylaxis, a potentially deadly allergic reaction that can have the same symptoms listed above, but may also lead to: * Difficulty breathing * Wheezing * Abnormal (high-pitched) breathing sounds * Confusion * Slurred speech * Rapid or weak pulse * Blueness of the skin (cyanosis), including the lips or nail beds * Fainting , light-headedness , dizziness * Anxiety, panic of a sense of "impending doom" * Heart palpitations   Generally, a shot of epinephrine as soon as anaphylaxis symptoms come on if the first, best and almost only treatment.