I think John C hit it exactly. That "die-off" can cause all sorts of pains and feel-bads.
To avoid most of that discomfort, take digestive enzymes while zapping and follow that up with a dose of the Lugol's Iodine [6 drops in water] NOT IF ALLERGIC TO Iodine -- to kill the eggs which are released from the dead parasites.
Also, the three herbs at least once a week and also L-Ornithine at bedtime and L-Arginine upon arrising. These wash out the ammonias.

I zap continuous sometimes for several hours. Doesn't hurt to zap 24/7, but NOT without the support of the herbs/enzymes/amino acids.

Bevpirt

What is the frequency of your $10 zapper?
The frequency of the $10 zapper from Luke as I know is
sometimes 50kHz,not 30kHz as in Dr. Clark's original
design (it could be some other frequency...).
Higher frequency doesn't increase the total power, but
it means more power is sent into the
human band. I did some computations and posted them on
the Zapper Forum comparing zappers with different
frequencies, etc. Result of the 10 dollar zapper was not
included there, so it is given below.

Each zapper can be characterized by it's frequency,
duty cycle, and amplitude of the square wave when
it is "high". With some simplifications --
assuming human body is pure resistance (5000ohms), amplitude
equals battery voltage--etc,
we can compute the zapper's total power and its power
in different bands. The result of our computations
show that most variations are not as good as Dr. Clark's
original design.

Example 1: Clark Zapper (f,d,a)=(30kHz, 0.5, 9V)
Total Power = P[total]=8.1mW Current: 9V/5000Ohm=1.8mA
Power below pathogen’s band P[<77kHz]=7.3mW 90.5285%
Power in pathogen’s band P[77kHz~900kHz]=0.7125mW 8.7963%
Power in human band P[1520k~9460kHz]=0.02763mW 0.3411%

Example 2: 10 dollar zapper (f,d,a)=(50kHz, 0.5, 9V)
Total Power P[total]=8.1mW (same as Clark's)
Low Frequency Power P[<77kHz]=90.5285% (same as Clark's)
Power in Pathogen's band P[77kHz~900kHz]= 8.3469% (Lessthan Clark's)
Power in human band P[1520k~9460kHz]=0.5686% (bigger than Clark's)

Is this (higher frequency, more power in the human band)
the cause of your headache ?
Or is it die-offs? I am not sure. Maybe other users
of the 10 dollar can offer their experiences?
We might all respond differently to the same zapper, however.

My guess is, if in the long run you feel better and better,
then it might be die-offs. If it keeps getting worse,
than it might be neurological damage. But in an early stage
it is hard to tell, and you certainly don't want to continue experimenting until you are sure it is neurological damage...
so be careful, perhaps try something else (herbal supplements,
cleases,...etc).

Beth