Can antihistamines cause fluid retention or would that be more related to the allergy itself? best ways to get rid of the excess fluid?

Not usually: in some instances, antihistamines and inhibit bladder emptying with back up of urine and kidney dysfunction. This can be serious, but it is rare and generally only occurs in susceptible individuals. Otherwise, antihistamines generally don't cause fluid retention. If steroids have been used, they can cause it. Benadryl can also worsen cardiac or thyroid disease with an effect on fluid retention.