What if i am concerned my hives are caused by an antihistamine in the first place? i.e. zyrtec and allegra (fexofenadine)

Very rare : Hives usually come and go randomly. So you may confuse an outbreak with being caused by something else (though not really). So best to see an allergist.

Hives: That is rare. Especially if suspect it from claritin, (loratadine) allergra, benadry, and zyrtec. Sometimes you have to realize that we can't find a cause but there is a long list of causes.

Go to allergy dr: Go to an allergy doctor and get tested. Usually if you take antihistamine and break out more you might suspect a reaction to antihistamine, but you won't know for sure unless it happens every time you take it, or you get an allergy testing.