Hi.. Thanks for the query. As per your complain severe itching along with bruising of chest and arms can be due to Allergic reaction that can be due to contact dermatitis caused with some skin cosmetics, soaps, detergents, poison ivy etc.. Any food or medicine can also be a cause.. Considering the same you have been advised the anti- allergics like Zyrtec and Benadryl.. I would suggest you to continue taking the medication as advised and along with it apply Calamine lotion.. In case if there is no improvement then you can consult an Allergist/ Immunologist and get evaluated.. You can be advised treatment accordingly which will include oral and injectable steroid and anti allergics.. Avoid exposure to the allergen once the cause is ruled out... An immunotherapy can also be advised to reduce the severity of symptoms.. Hope this helps.. Regards..