I radically removed sugar (aka "sucrose", which is half glucose and half fructose) and removed fructose from my diet for a couple of weeks now, but continued to consume dextrose (aka "glucose"). And what do you know,
Now I reintroduced sugar for a short time (ate some fruits yesterday), and today my muscle problems returned. And with it came the indescribable "weird" feeling.
As fructose (and fructose in sucrose!) is metabolised in the liver – and unlike glucose, only in the liver – this would indicate a problem with my liver. Which would tie in with the increased GPT value (aka ALT) I had after eating dairy for 10 days in March 2013.
So liver sub-clinical liver problems can manifest with muscle pain. What do you know.
(How I wish I could go back to 2010 and correct some bad decisions I made made back then in order to battle my ME/CFS. I had to find out all by myself and followed the low-carb idiots, and the Paleo beef idiots, and the ME/CFS pathogen idiots – and got seriously hurt as a reward.)
[Addendum] If I take Ibuprofen (commonly known as "Advil") for migraine/headache, then my migraine goes away, but I get both muscle pain (albeit muted) and similar "weird" feeling (even at "only" 300mg Ibuprofen) – more indication that actually I have a liver problem.