I get a similar feeling when I stay in one position for too long. When I got MG, but before starting to take Mestinon this happened all the time and much more extreme than it does now. It felt uncomfortable to lay down, it is hard to describe what it felt like, kind of tingly, not really painful but more uncomfortable, and whatever part of my body I put my weight on was more susceptable to being my weakest muscle.

I don't have actual pain with my MG, but this may be because I generally have a high pain tolerance.

Many of your symptoms sound like MG, but I think there is also something else, although MG seems to occasionally cause symptoms that are not typical. If you have MG, maybe your symptoms are not typical because you also have Fibromyalgia (or another autoimmune disease since the risk of having/getting more than one autoimmune disease is high one you have one of them). If the doctors have "ruled out everything" but haven't tested for MG, I think it would be worth testing for, but of course I am not a doctor.

One of my least favorite things about MG is that I usually look normal and therefore am expected to be normal. It is hard to deal with these expectations.

I hope you figure this out soon.