If sitting for long periods of time adds to the pain, see a chiropractor. It may be a tilted sacrum from too much exercising. It can be adjusted.
If you have no leg pain you don't have nerve compression. MRI Scan lower spine will rule out anything sinister. It sounds muscular to me.
Try ice packs to painful area rolled by heat pack.
Hope you get relief.
Dont worry ,,,
Consult a neurophysician ,or first get an mri of spine with whole spine screenin ,then meet the doctor .
Its better to clear doubts and to take right medicines .
Good luck .
I am not a doctor or expert ,its just my opinion.