Widespread itchy rashes with pimple-like spots and swelling can be caused by several conditions, most commonly an allergic reaction, heat rash, a fungal or bacterial skin infection, or a condition like Urticaria, especially if the itching is intense and the rash comes and goes. Involvement of areas like the underarms suggests that sweat, friction, or infection may be contributing. Scratching can worsen the rash and lead to marks or secondary infection. It is important to keep the skin clean and dry, avoid tight clothing and new soaps or products, and try not to scratch. You should see a doctor for a proper diagnosis, as treatment may include antihistamines, antifungal or antibiotic medicines depending on the cause. Seek urgent care if you develop swelling of the face or lips, breathing difficulty, or fever, as these may indicate a more serious reaction.