Allergy to ragweed is not ordinarily a cause of hives, but an individual who is prone to allergic reactions might be at increased risk of hives. Finding the cause of hives requires some "detective work", with careful consideration of all recent exposures, including food and liquid ingestion. Stay with the anti-histamine until the hives resolve. If they continue to worsen, you may need a short course of oral corticosteroids. You can safely ignore slight, quickly resolving swelling of the lips but greater swelling, especially if accompanied by swelling of the tongue should be considered a medical emergency and should be quickly treated with steroids, such as prednisone or cortisone. Please read our hives (urticaria) information at http://www.nationaljewish.org/disease-info/diseases/allergy/about/conditions/hives.aspx to learn more.
Sorry to say, but the cause of a large percentage of cases of hives, is never found. This may change in the not too-distant future as hives have become a disease of great interest to many medical researchers.