I wonder if your Vit D levels are low? This nutrient is getting alot of research attention and this new video is very interesting and made for the layman:
http://neurotalk.psychcentral.com/thread92116.html

I would also watch your insulin levels, and find out if you are getting borderline with that. A fasting insulin level will show more than a fasting blood sugar (which may not show creeping increasing problems with energy production)

Also a HbA1C test will show if you are having elevated blood sugars during the day. When this happens, fatigue can become considerable. As we age many people develop this problem.

If you feel specifically that you have MUSCLE weakness the supplement acetyl-l- carnitine may help. It works with the energy parts of your cells(mitochondria) to create more energy than with glucose alone. You can try it at 500mg a day to start, and if you feel stronger, you can increase to about 1-2 grams a day, in divided doses.
(2 to 4 capsules)

Another cause for fatigue when you suddenly improve B12 status is that when you are anemic like you were, Pearl girl, and the body suddenly can restart to make red blood cells, it uses up potassium in the blood serum to make the new cells. When that happens fatigue and sore muscles may occur. So you should try to eat high potassium containing foods to provide enough potassium so you are not robbing other parts of the body until the red cells are brought back to normal.
The average red blood cell lives about 3 months, and is replaced. So this process of repair can be stressful for a while.