I'm certainly not a doctor, and the rest of the forum doesn't give doctor advice either. But it sounds like an allergic reaction, like tongue and throat swelling, to me. It doesn't take much for you to notice it, less than for someone else to see it.
If this gets worse or you get really scared - go to the ER! The weekend's here and the offices of most doctors will be closed, or I'd say go back there.
Did your doctor say anything about taking a different antihistamine? Alavert works differently and my doc says I can take it in addition if I need it.
It's possible you are having a reaction to the Benadryl also. Some people do! Is the one you have liquid or liquigel or a colored tablet? Since you reacted to a dye, you could be reacting to dye or flavoring in medicine. I would try a different antihistamine.
I'd also try shampooing as much dye out of my hair as I could, and making sure to use a shampoo without colors or scents. I'd also avoid any sort of food or beverages with coloring or flavors added.
Can you get a list of ALL the ingredients in the hair dye? You'll want to discuss them with your allergist.
I think you've opened Pandora's box. We'd sure like to hear more from you, how you're doing, what ingredient is the culprit ...