Muscle pain

I was just diagnosed with MG in February and my initial symptoms were difficulty chewing and some fatigue. I have managed fairly well on a dose of 10mg prednisone a day and 60 mg Mestinon 3x daily. I really slacked off of the Mestinon due to the frequent bowel movements it caused. In the course of my job I drive to different customer locations that are as far as 5 hours away. Many of these locations are not near a major interstate highway so no rest stops along the way. Anyway, recently I have been noticing that my legs feel week when walking say as far as from the Walmart parking lot to the back of the store. About a week ago I began experiencing increasing pain in my lower back that would shoot down the back of my thighs. I went to my primary care physician and x-rays don't reveal any spinal issues. I was given some Tramadol to help with the pain along with a muscle relaxer. This morning the pain seems more in my buttocks and thighs and now my calf muscles hurt as well. I normally shower and shave first thing in the morning but the pain makes it an excruciating experience. Standing is the worst thing. I have a physical therapy appointment set up for next week but I suspect this is an MG issue and the therapy probably won't help. I also have Rosacea and had been prescribed Doxycycline for it. I had stopped taking it as it can be found on some lists to avoid for MG patients. I restarted taking it about 3 or 4 weeks ago due to a skin breakout near my left eye. A light bulb went off in my head this morning and I am discontinuing the Doxycycline as I fear it might be the cause of my increased MG symptoms. Does anyone else experience severe muscle pain that they think is related to MG?