I'm sorry you're going through this! It sounds like you've been to several doctors & they've done exams, bloodwork, x-rays, etc, all coming back perfect. I would say it does sound like anxiety. And anxiety can cause you to lose your appetite, which obviously will cause you to lose weight. I'm not an expert on weight loss, but years ago when I went on a special diet (which was a healthy one, not some crazy fad one), I lost like 10lbs in about 2 weeks. It was basically cutting out dairy, cutting back on sugar & eating more fruits & veggies. So I don't think 10lbs in 2 weeks is a bad thing, but again, I'm not an expert on this! If you keep losing weight without trying, then you should definitely talk to a doctor.
If you're still having back pain -- or any muscle pain, for that matter -- I highly suggest googling stretches for whatever area is bothering you. (This is one of the few times it's ok to google something!) I go to google images for this. I was having back stiffness in the middle of my back & my upper back for months. It was awful. Stress & anxiety definitely made it worse. I also found out I had a pinched sciatic nerve, which sometimes caused lower back pain and my right leg & foot to have issues (mostly pins & needles feeling). AND I often have tight muscles in my neck & shoulders, especially when I'm stressed. I had x-rays & bloodwork done, too -- everything came back fine. One of the doctors I saw suggested googling stretches for the sciatica (or seeing a physical therapist if necessary), so I did just that. Wow! Let me tell you, after doing this routine for about a month, I'm doing so much better. The stretches definitely help the sciatica & as a bonus, my middle & upper back are feeling MUCH better. I never thought that nagging stiffness in my back would go away & was sure it was part of some awful disease they missed, but it's finally almost gone! It flares up if I'm really stressed or if I sit too long, but then I go do my stretches.
I also do a short series of stretches for neck, shoulders & arms. Doesn't take long to do. Great for loosening up tense muscles. I started doing it every morning to loosen up, then I do the sciatic stretches. Now I feel like crap in the morning until I do this routine & even though my mornings are hectic, I make sure to do this because I know it's something I need to do for ME to keep myself going. Anyway, sorry for the long reply. Let me know if you want links to any of the stretches. Good luck!!!