Hello, I can understand your concern for sudden occurrence of rash on knees and elbows and rest of body. After reading your it appears that you are suffering from Urticaria . Urticaria is a common condition caused by bursting of allergy producing cells known as mast cells due to certain aggravating factors present in the environment. As the chemical inside them is released it produces a rash which you have mentioned. There can be multiple factors producing this condition which are included in day to day life and some times it is really difficult to find one exact cause. In your case you are correlating it with exercises, hot climate,spicy food. So it is better to stay away from the offending factors. 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. Do not scratch or rub the skin as it can lead to further aggravation. Take care.