I am also fairly certain it is NOT an M.E specific thing - in my quest to find out what is worng with me I have hung around on:
MS boards,
Lupus boards,
Thyroid boards,
Lyme boards
MCAS boards
They all discuss something VERY similar. But whether the subjective experience of crashing or relapsing after too much activity, either immediately or delayed is the same in M.E as it is in these other illnesses or different as perhaps shown on exercise testing I do not know. Do we know if people with these other dx have ever done the same two day exercise tests?
I can maybe address this. I am diagnosed with MS. I've had MRIs, a spinal tap, and bloodwork that all showed without a shadow of a doubt that I fit the criteria for MS. My first symptom was optic neuritis, a textbook first MS symptom. However, my MS seems to be atypical as my symptoms are much more closely aligned with CFS than with common MS symptoms. Additionally, in comparing my most recent MRI to the one taken 7 years ago, there has been no increase in brain lesions and I have not been on medication during that time (so lack of increased lesions is very unusual). My main issues are: extreme crushing exhaustion, impaired memory and concentration, POTS, unrefreshing sleep (often feeling more tired when I wake up than when I went to sleep). My neurological symptoms are minor and I have no difficulty walking. As an aside, I also had chronically swollen glands and a history of several major viral and bacterial infections.
I also occasionally get PEM, though I don't get it often, and it has never lasted more than 1 day. Once, it happened after walking 10 minutes (I had a feeling that something wasn't right, ignored it and walked anyway, then crashed and was immobile for several hours.) A few other times I got PEM after doing non-strenuous exercise that I had been doing regularly with no previous problems (for me, PEM usually lasts anywhere from an hour to a few hours and often feels like I've been hit by a bus, involving the muscles in my whole body, not just the ones being used - so for example, after doing lunges I would get 'dead arms' and not be able to hold a glass of water).
I also have low blood pressure and have issues with POTS that seems to come and go - when my BP is around 90/55, I am often dizzy, lightheaded, have trouble standing and bending down. Then I have periods of time where this issue mysteriously disappears, and my BP tends to be in the more low/normal range (100/60).
\
You could potentially meet Canadian Consensus Criteria - A patient with ME/CFS will meet the criteria for fatigue, post-exertional malaise and/or fatigue,sleep dysfunction, and pain.
You could potentially meet The CDC (Fukuda 1994) Definition for Chronic Fatigue Syndrome -
The concurrent occurrence of four or more of the following symptoms:
substantial impairment in short-term memory or concentration;
sore throat;
tender lymph nodes;
muscle pain;
multi-joint pain without swelling or redness;
headaches of a new type, pattern, or severity;
unrefreshing sleep; and
post-exertional malaise lasting more than 24 hours.
And again, to further complicate things, I fit both criteria for CFS even though I have MS.
I think the difference though is that people with ME/CFS don't just crash for a couple of hours or a day or two. For some of us it can last for a week to weeks at a time. I definitely think that in order to have a diagnosis of ME/CFS you have to have PEM.
My husband has what I consider 'true CFS' and he gets extremely bad PEM that usually lasts for several hours/the rest of the day. His PEM crashes don't typically last for longer, though he does have weeks or months of feeling especially crappy.