I saw this thread a few days ago and had to have a think about it.
I think primarily that I know I have something remitting/relapsing. Because while I never feel 100% (shoot I'd be happy with 70%), there are 'flare ups' that seem to come on over a period of a few weeks or months that lay me out flat. Then I stop everything and focus on quiet and peace and just flat out laying down as much as I can; which takes a month or two or three before I'm back on my feet and as long as I'm REALLY REALLY careful and pace myself, so far, I've come back gradually to my half-life.
BUT every collapse has been a bit different. I'm still not 100% sure I have ME/CFS, but it's the closes most plausible thing I can find. I do meet all the criteria for it I've ever found from CCC to the new SEID, etc. But I think I may have some mild hypermobility and/or EDS or some other issue that might be undiagnosed. AND I have previously been diagnosed with B6 toxicity (which I honestly blame for my first crash and either I'm not well yet; or it was chased quickly by ME/CFS). I've debated a few times if it might be MS but it certainly wouldn't be 'typical' MS and I've had about 6 brain MRIs from doctors looking for stroke or MS and nothing found there (and get ignored when I say 40% of people with MS don't have brain lesions).
Year-round symptoms include frequent diahhrea, food intolerances, smell intolerances, increases in allergies, dizzy spells, PEM if I exert beyond 'current envelope', difficulty standing any length of time, muscle fatigue, frequent muscle pain, frequent muscle 'failure' where I'll be walking and one leg will spend about 2 seconds saying 'nope' or lifting something and an arm muscle goes 'nope'. SEVERE can't stand being vertical or even in a chair for another second fatigue as sitting requires muscle and there is so much pain from all muscles that only laying down at full 'rest' alleviates it.
I'm sure there are more, those are just the ones that bother me the most on a regular basis.
Major Flare up #1 - 2005 - dizzy spells and loss of sensation on the right side of my body and severe pain on the left side of my body built and built until suddenly being upright brought on such severe nausea within about 10 minutes, I couldn't sit up or stand up without pitching to the ground. Laying flat helped the world stop swirling and helped with the nausea. Pain was still bad, but flat out on my bed meant no work anyway. It took about 3 months for the constant dizziness to back off and let me get back on my feet. The pain died back to the aches I experience now. The sensation loss started to get restored years later after starting some treatment for Methylation.
Major Flare up #2 - 2012 - sudden onset dizziness - laid out flat for a week. Thankfully this one passed much more quickly. Some rough guesses were that something got damaged and caused flare up #1 - which if one spot out of the three in the brain that control the body's 'placement in space' got damaged; it usually takes 3 months to recalibrate (docs' guesses); but that if suddenly that 'specific' spot started to work again? It would cause the body to go through another recalibration process, but that it might not take as long since it's had all three functional in the past, it's just a matter of retraining the brain to recognize all three inputs. My guess ? Severe POTS both times.
Major Flare up #3 2014 - started to develop tiny painful swellings all over my arms, legs and trunk (about the size of a quarter). They were forming in the muscle tissue areas - none around joints, not just at skin or just under the skin... no discoloration. Doctors couldn't begin to guess... but I discovered if I marked the 'spots' with a marker that every night some of them would disappear and every day more would appear. I spent seven weeks laying down and ended up with them all disappearing. At one point I had a theory around lymph nodes and drainage issues; but I got better before I could test any theories. To this day, if I have a busy/active day I get a few 'lumps' that will go away with rest. So, they've become my pace monitors right now.
There have been about 20-30 minor flare ups that feel like PEM that I haven't kept track of. But these are the three where I had to consider short term disability. Thankfully the second one was short enough that I didn't end up needing it. Even more fortunately I'm eligible for short term disability and long term disability through work. AND I can work from home to help reduce PEM when I'm on a down-ward trend and my 'window of activity' is shortened.