Whitish, itchy, bumpy Idiopathic hives turn flat & red after taking antihistamine. Is that normal? Maybe problems with mast cells / blood vessels?

Yes and no. Hives can at times behave this way but if you only have hives then it is unlikely that your problem is a mast cell disorder. Be sure you use a moisturizer immediately after bathing since dry skin can cause itchy bumps. You may also try upping the antihistamine dose to 4 x daily. If still not helpful , it is time to see a dermatologist or allergist.