Most of the times the causative allergen remains unknown and thus fall with the terminology 'Chronic Idiopathic Urticaria'.However this is no reason to trying to identify a cause.
The most common food allergens are nuts, dairy, gluten and soy. But some people are allergic to different fruits also. It takes a lot of work to avoid gluten and dairy, but it is worth it. Get tested for Celiac disease.