Constant red face and arms with flushing, could it be gluten sensitivity or dairy?

[Silversunfan]

I'm almost eighteen and I've had a red face for as long as I can remember. My forehead and nose are fine for the most part, but my cheeks flush (as in a burning sensation and turning a bright red/purple colour) often, especially in the afternoon, and I have blotchy red patches along both jaws, often with hives.
On my arms I have keratosis pilaris, which I understand can't be treated, but I also get blotchy rashes very frequently and they can be seen any time I'm exposed to water.
When I take antihistamines I have some relief, but not much - the only time my face is completely clear is late at night, after the sun has gone down. I usually don't wear makeup and when I do I only use mascara, as I don't want to put anything on my face to irritate it (and nothing is strong enough to cover the redness).
I googled gluten rashes, which can occur in people who are sensitive to gluten but don't have full on celiac disease, and I'm wondering if it could be that. My skin improves a lot if I fast, and I believe it's dietary. I researched dairy allergies but they usually don't produce allergies. Eating fruit makes my rashes worse.
I've had this my whole life but it's become more and more extreme every year, and I can't deal with it any longer! What could I be allergic to?