Muscle pains all over. Underlying cause?

I have been suffering from muscle pain/burning sensation under the skin/ muscle weakness and some degree of pain in joints especially right behind the heels (achilles tendon) for some years now. It feels like all my muscles are weak and tire easily and there's always slight burning underneath the skin. Also, I have gastric discomfort right below the breastbone and have been taking medicines for the same. There's been a few times in these years where the muscle weakness has been very debilitating. No swelling or redness anywhere and no morning stiffness. Tests of Rheumatic panel are all normal except for ASO titre which is at 440. I get all sorts of other fleeting pain and cramping in the body too. All vitamin, minerals, vitamin D/B, iron and other basic tests are normal, thyroid normal. Taking a healthy diet. I do feel very cold in cooler weather and don't tolerate winters well. Have tried treatments but there's no answer to this. What could be the underlying cause?