What you describe corresponds pretty closely to what we know physiologically. The euphoria is consistent with endorphins. I do have to admit that I do not know of anyone who has confirmed this by blocking the reaction with drugs like Narcan that prevent the binding of endorphins to their receptors. I suspect that the pin-point pupils may be do to adrenaline due to the variety of mediators of acute inflamation (histamine, seratonin, prostaglandins and perhaps some cytokines) released from the welts plus adenergic stimmulation. If this is true there the pinpoint pupils should be accompanied by rapid pulse and respirations.