After trauma to the back, arthritis (degenerative disease) is common. You need a good neurologist exam and imaging, preferably MRI of the neck and back.
A broken coccyx (tailbone) often will flare up later in arthritis. I doubt this pain has anything to do with the spinal injection 20 years ago, but with the fracture itself. I, too, had a fractured coccyx, at age 12 and my first bout of arthritis in it at age 30. You describe all the symptoms to a T! The pain on sitting, severe pain will bowel movements, bearing down, coughing, snezzing, squeezing your anus tightly. It's because all of these thing cause the local muscles to pull on the arthritic spot at the very base of your spine. You need a "Tush Cush" a wedge-shaped pill to sit on that has a hole cut out for your tailbone. It will give you some relief. You can also buy a piece of 2" foam and cut a hole about 4-5" across to make one yourself.
I'm sick today, but want to discuss your dizziness which could easily be related to cervical spine disease or other things. But don't use this forum to put off a trip to see a neurologist!