I think you could do better with active B12 orally--- methylcobalamin.

5mg a day on an empty stomach and your levels should go above 1000 within 3 months. Maybe higher.

People who have been as low as you were for a long time develop nerve damage. The B12 is transported into the CNS based on a concentration gradient in the serum. If you are really low, your nervous system would have been deprived too.

Your pain, etc may be the nerves "waking up" and trying to reconnect.

Have you looked this up? Combined degeneration of the spinal cord?
http://en.wikipedia.org/wiki/Subacut...of_spinal_cord

In people who have the DNA mutation that prevents methylation of B12 (your injections are cyano and need to be methylated) and methylation of folic acid...this neurological deterioration may occur.
Here is an article explaining the folate aspect:
http://docs.google.com/viewer?a=v&q=...Wj1iSwgItowcFQ

The mutation is called MTHFR mutation and there are now DNA tests for it. Estimates are that about 10% (some researchers now suspect higher numbers) cannot activate B12 or folate properly and need these supplemented by the methyl versions.

Both are available OTC as Metafolin and methylcobalamin.
An RX vitamin is also available with higher doses of methylfolate and it is called Metanx.

It appears you are not responding properly to your injections and it is a clue to me that you need other interventions.

I have medical links on my B12 thread here, so do please read it. Many doctors do not understand B12 well, they don't keep up with the new products or the research, so when that happens patients have to step in for themselves.
http://neurotalk.psychcentral.com/thread85103.html