Be more careful.

20 minutes straight isn't the right amount. 7 minutes is the maximum at a time that will do any good. you need to take 20 minute breaks between 7 minute sessions, and you need three of them in a row. this takes about an hour. the first time gets larger parasites, the second time gets remaining microbes, and the third time gets remaining viruses. one time at 20 mins would leave a bunch of bacteria, fungi, and viruses still alive.

the adverse reaction when you weren't expecting it, coinciding with not following correct procedure, looks to be evidence that the zapper has sound theory behind it, and is not simply a placebo.

zappers can only kill what they can reach. candida can hide (and thus reinfect within a few hours) but not while it's doing it's major damage. your own immune system is the most powerful weapon to get rid of it completely, but the zapper can help reduce the burden from it.