Are you putting the cayenne when you pre-mix your drinks in the morning? Cayenne "blooms" as it sits in the liquid and gets hotter!! If you put the cayenne in just before you drink, you should find that you are able to tolerate more and more. I typically start my day 1 with a little less than what the recipe calls for and over the course of the next few days steadily increase to 1/4 tsp. per glass.
I premix my lemonade in the morning so that I can take it to work with me, and I carry around a little jar of cayenne and a measuring spoon.
I hope the next day or so work out better for you! :)