This is an interesting topic, so thanks for bringing it up. Here is a summary of my relevant experience on this topic:
A few people have reported fatigue after using ICES-PEMF. This seems to be similar in some ways to a Herxheimer reaction, but probably different mechanisms of course. But similar in the sense that the treatment can cause release of toxins that can cause an initial, transient, negative response. In the case of PEMF, it may be that reduced inflammation initially causes the release into the system of something that had not been clearing properly (imagine, for example, clearance of a swollen lymph node as it suddenly begins to flow normally again, just as a mental image). Once the initial clearance has happened, the negative reaction seems to be reduced.
Some people have reported to me that they find ICES-PEMF to be extremely relaxing (as opposed to causing fatigue). Typically this is the case for people with postural problems from severe chronic lower back pain. The swelling can reduce so significantly, and the pain and postural tensions/imbalances can disappear rapidly, so much so that some people tend to fall asleep in the middle of the day. For at least one person this was enough of a problem that he decided not to use ICES-PEMF while driving, probably a wise choice.
A third phenomenon relates to nerve/brain entrainment. Even peripheral nerves can be entrained by the externally-applied pulses. For some people this can have unpredictable effects.
It may just be that you are wired up, so to speak, that peripheral nerve inputs have an out-sized influence on your CNS. That may be the case. I experience that myself to a lesser degree, but I can definitely sense it, even when using ICES-PEMF on my feet.
So, with all that in mind, starting low and slow is probably a good strategy for you, as well as experimenting with the pulse protocols and taking notes, to see if you can detect some sort of pattern that would give you a hint as to what the role of pulse pattern and frequency is in your individual response. In my case, I find Omni-8 to be slightly more effective for reducing pain, but also slightly more fatiguing. The differences for me are very slight, but I sometimes notice them.