Fluctuating B12 levels

Hi

After suffering with post viral syndrome for over 12 months (parvovirus in spring 2010), viral thyroiditis, and subsequent chronic fatigue, chronic pain, I discovered I had B12 deficiency (149 pmol/L). I was put on 1200IU oral tablets of B12 for 1 month to see if I responded to them before resorting to injections. After I started taking them I noticed less fatigue and better concentration. Pain levels, especially pain along my forearms that are prehaps nerve pain(?) are reduced (but not gone). However when I had my level tested again last week (about 4 weeks after starting the tablets) I am now in the 700's and over the normal limit. I have been told to stop taking them. The Dr is also surprised that I have responded in such a way and that such a big change should not occur over such a short time span. My symptoms have already started coming back. I'm also wondering why he is concerned that it has shot up so quickly?

Any help and advice would be appreciated.