Crimpjiggler-it sounds to me like the beginning signs of opioid withdrawal.
I'm on opioids myself, have been for a good few years for a knee injury, first codeine, quickly changed that to DHC, and just very recently to a combination of oxycontin and instant release oxy (bloody finally, something that actually works). What actually got me the oxy script ironically enough wasn't that the DHC wasn't enough to lessen my pain enough for me to walk without a stick (I'm 25 and shouldn't have to worry about that for 65-75 more years) but rather, that it doesn't last nearly long enough for me to sleep and not wake up in withdrawal, and since I'm utterly shite at keeping time (yay, thanks a crowbegotten bloody bunch dyscalculia) trying to stick to a regular dosing schedule would be an exercise in futility and frustration.
What served as my clock, was a combination of pain getting worse again, and opioid withdrawal. First thing about it that I ALWAYS experienced was akathisia, which is exactly as you describe 'restless body syndrome'. Have you ever experienced bona fide opioid WD? (by bona fide I am not belittling what you are going through as somehow 'not bad enough to be called real WD', but rather experienced something you can be completely certain was indeed opioid withdrawal?
Whilst I wouldn't rule out getting your ferritin levels tested, vitamin deficienci looked for, and tests done for other things which can cause akathisia just to eliminate the possibility of anything more serious it wouldn't surprise me if what you are going through is actually withdrawal of limited severity. I am unsure of your experience, or lack thereof, with opioids, so don't know if you would see withdrawal for what it is if it jumped up and bit you, which of course, is precisely what opioid WD is notorious for doing:/
If you have never gone through it (before), the initial symptoms are akathisia, piloerection thanks to the sympathetic overload going on, although given you take a sympathomimetic drug, i.e the dexedrine that may occur in the absence of WD, increase in libido (opioids suppress testosterone), sweating and being physically shaky.
You mention taking magnesium with a cocktail of other stuff, which helped. I think it quite likely was the Mg that did it, due to the fact that NMDA receptors have a large role to play in tolerance, and NMDA antagonists can attenuate tolerance to opioids, (or reverse existing tolerance?* *I just took my oxy, and went from mild withdrawal to a state involving significant ptosis, so can someone else check up on the tolerance reversal for me, I really don't feel much like digging through journals to find out right now)
Is it made worse if you lower the dose of opiate, or cease it entirely? if it is, then I think you just found the culprit.
Although....incidentally you mention autistic people stimming, and make the comparison to placing yourself in a position imposing significant physical stress on your body and that alleviates the akathisia. I am autie, I rock, spin round, exhibit the stereotypical hand-flapping stereotypy and it certainly does no such thing if I am in withdrawal. Never had akathisia until starting on regular opioids on rx, I used recreationally occasionally before that with no problems but I knew what was coming when I signed the devil's pact.
I agree with what others have said though, you are in for a world of hurt if you keep on the usage pattern you described.think what you deal with now is bad? it gets worse. Its like comparing a kick in the bollocks with having your eyeballs routed with a dentist's drill. And if the akathisia ISN'T caused by WD, then make that a drill with a dog-shit coated, syphilis infested drill bit.
Off topic \/
You are barking up completely the wrong tree when you spoke of the reasons and motivations for auties and aspies stimming. Many, or more probably, most, of us perceive the world far more intensely than our NT counterparts, sights, hearing, olfaction, tactile sensation, emotions. A lightbulb that is completely normal for someone with neurotypicality could be blindingly intense for an autie/aspie who is photosensitive, and our emotions, the high points can be intensely so, but the same unfortunately goes for the lows.
Stimming is an outlet for that, at least in parts. It is a method of exerting control over one's sensory input, and for some, an outlet for when things get too much in the way of overstimulation. And it feels good, feels like the completely natural thing to do. To me it would feel very strange, and very wrong if for some reason I suddenly no longer felt the desire to stim.