Hello, I understand your concern for severe hives. Hives or commonly known as Urticaria is a condition caused by bursting of specific allergy producing cells known as mast cells . It is due to certain aggravating factors present in the environment which causes bursting of these cells and release of chemical inside them is released it produces a rash which you have mentioned. There are be multiple factors producing this situation. It is really difficult to find one exact cause. The long term effective treatment for this is by oral anti histamine like loratadine under medical advise. It is one of the drug which is not having any side effects and can be taken for long term usage. For the same I prefer to give an antihistamine such as cetrizine or levocetrizine or loratadine for two times a day for few days. The most important part is avoidance aggravating factor which you can notice by making a diary of causes to rule out the most probable causes within two hours of occurrence. All the best.