Finding a "cause" for many with acute urticaria in most patients is not a reasonable expectation. It can be caused my infections, medications, other causes... but is generally idiopathic. If you have airway symptoms seek immediate care.Otherwise, see https://dermnetnz.org/topics/acute-urticaria
That looks very uncomfortable. Im NAD, but have experienced a similar looking rash everywhere as well. Your doctor may be able to help you, but I do know that putting ice packs on the particularly itchy or painful areas really help to relieve the discomfort. Just wanted to give you ideas. Hoping you feel better very soon.