Based on the symptoms described, such as red bumps on the arms with slight itching, and considering that you were exposed to the sun while working on your balcony, it is possible that the reaction is due to a mild sunburn. In cases like this, it is recommended to apply cool damp compresses, calamine lotion, or aloe vera gel to relieve itching. Make sure to keep the affected area clean, especially if the bumps burst. If the issue persists or if you develop other symptoms like fever or difficulty breathing, it is advisable to consult a doctor.