There are many reasons for sudden hives, as follows: 1. Allergic reactions caused by drugs, which may be due to contraceptives or cold medicines, antipyretics, antidiarrheal drugs. 2. Solar urticaria caused by ultraviolet radiation from the sun. 3. Cholinergic urticaria caused by sweating, exercise, or after drinking a lot of coffee in young adults. 4. Allergic reactions are caused by the inhalation of dust mites, dust mites, or dust, or by the consumption of beef, mutton, seafood, eggs, or milk. 5.