A: Histamine contributes to stomach acid, so that might be related. Chronic urticaria is often associated with a parasite called Blastocystis hominis, which is also associated with IBS and Hashimoto's. I have an article on Blastocystis hominis regarding its connection to hives, urticaria, Hashimoto's, and IBS. It also causes food sensitivities. So, I'm not sure if it's the antihistamine that's contributing, or if the parasite infection is the underlying cause of your urticaria. I would take a look at that.
One of the protocols I recommend for that is Saccharomyces boulardii. Many people have been able to address it with just that probiotic, taking it twice a day for 60 days. There are various strains of it and in some cases more intensive protocols may be needed. So, I have more information about that in my article as well.