Have you checked your urine and saliva PH? Maybe your PH is to acid and that is why you react with the pads.
You could buy the optional Copper Pipe Add-On Kit for $10.00 for your zapper. Then you could see if you have the same reaction with that you have with the pads. If you don't then maybe you do have an allergic reaction to the sticky pads as Marti said.
I would think that by now if it was a reaction caused by parasites that it would stop making the red bumps because they would be dead.