OK thanks. Some things I can think of that might cause that sort of pain might be:
- Muscle spasms/tightness
- Kidney stones
- Prostate problems
- Some other kind of internal organ problem
- Lack of exercise and active lifestyle
You can fix muscle spasms/tightness by starting to exercise again, making sure you stretch your whole body on a daily basis and begin to lead an active lifestyle again (the sitting at a desk needs to be balanced with exercising).
As for internal organs, I really hope you don't have any of these, but might have to get your kidneys and prostate checked out. Both of those are known to cause unpredictable back pain.
I don't know anything else really, but please do get a few more opinions, and see if what I suggested might be of any use to you.