I get this symptom as well, particularly in the proximal muscles (upper thighs, sometimes upper arms). It has been so bad at times that I have been unable to walk. For me sometimes it coincides with reproductive system issues (e.g. PMS, ovulation, etc.) but other times just flares with other symptoms, as part of being run-down. I use essential oils and CBD topically to help, and make sure I have been getting enough iron, calcium, potassium, and magnesium. (Be sure to take iron and calcium at different times of day, if you supplement them.) I have been diagnosed with viral myositis before, because the first time this happened I had recently recovered from a bad virus, but I'm not sure that was a particularly useful diagnosis.
If anyone had tips for moving this symptom along when it strikes, then please share!