Muscle Weakness and Severe Pain

Sex: Female. Age: 18. Height: 5'8. Weight: 170. About a year and a half ago, I started experiencing strange pain in my right knee, eventually causing similar pain in my left knee. It was difficult to walk from the pain and went to physical therapy for it. For awhile the pain was lessened, but around the 4 month mark, I started experiencing weakness in my legs that made it hard to walk and started causing pain in my thighs. Around month 8, I had pain in my legs often and the weakness/pain began to affect my arms as well. Currently, at month 18, I require crutches for short excursions outside the house and a rollator for longer trips with more walking. Weakness ranges: sometimes not too bad, I barely need my crutch, other times it's so bad that I can't even stand while leaning heavily on a wall and have to sit down for a few minutes because of how much my muscles are burning. Can become incredibly sore, particularly in the hips and back, after walking or even just standing for too long (amount of time depends on how my body/weakness levels are that day.) Pain description: Can range in intensity from 1-9/10 (can also describe as 1-5\[not a problem\] to 6-8\[visibly in pain, probably swearing under my breath at work, near children who I should not be swearing by\] to 9+\[actually crying\]), often dull but can be sharper, most common in front of thighs and upper arms but can be in other parts of arms/legs and back. Plus, I get really sore, as stated above. Actions taken so far: - Physical therapy: Didn't help weakness, unfortunately. - MRI of neck/head: Nothing found. Potentially significant info: - Uncle had Type 2 Myotonic Dystrophy. Neither me nor my mother have had genetic testing done. Doctor doesn't think it's necessary. - Currently doing screening (ultrasound/MRI) for Endometriosis because I suffer from many of the symptoms of it. - Ex-competitive dancer, I was very active before this started affecting me. (It's been hard not being able to dance/be as active tbh.) - Have been successfully losing weight and my issues have only continued to get worse despite the weight loss (but I'm still going.) (Caloric deficit. Have been losing 1.5 lbs each week. If that matters?) Could this be related to potential Myotonic Dystrophy or Endometriosis? Could this be myositis (a condition that has similar symptoms to what I'm experiencing)? Could it be something else? What further steps should I take? Any advice would be appreciated!