Severe back pain for over three weeks, x-ray found mild arthritis and an old fracture. very bad leg pain tingling in both feet. upper back pain also, worse with sitting, improves when active. blood work normal metastic lung cancer likely? als?

Unlikely arthritis: Unlikely metastatic lung cancer unless you HAVE lung cancer. Unlikely ALS by the way you described the symptoms. However: how do you know the fracture is old? Also, how can you have arthritis at a young age with normal blood tests? Certain lower back pains improve with activity. If you have tingling in both feet, and that is also new, then you need a spine doctor and a spine MRI to start