The area you locate the swelling can be of any nature. you say that it is painless, fixed & ball shaped.
As you have expressed - it may be Non inflammatory swelling. So swelling may be arising from skin,subcutaneous fat, muscle, fascia & bone. To find the location you need MRI
Scan of the lumbar & sacral bones. The result will clearly indicate whether swelling is arising from bone or superficial to the bone. If superficial to the bone the possible plane of the swelling. After that the Dr will decide whether FNAC or Biopsy
to be done. Get the opinion from General, Orthopedic & Neuro-surgeon and initiate the treatment as early as possible.