With a B12 that low, it will take time to recover. Often there can be temporary shifts in potassium in the blood. This is because B12 stimulates the formation of new red blood cells, which gobble up potassium. So temporary lows, should be dealt with by eating foods high in potassium.

Potassium and magnesium work together in the muscles. You may need some additional magnesium too. Go for high mag containing foods like almonds, beans, oatmeal, yogurt. You can take a good chelated form as a supplement, NOT OXIDE however. Aim for about 1/2 the RDA which is 200mg elemental daily. Muscle issues, tightness, etc often respond to fixing magnesium.
Here is my magnesium thread:
http://neurotalk.psychcentral.com/thread1138.html

normalizing potassium and magnesium will help blood pressure, sleeping, and muscle tension.
This excellent food link will tell you what is in your food.
http://nutritiondata.self.com/
For example, a V8 juice 12oz will deliver just over 800mg of potassium, which is a goodly amount.
3oz of almonds = 270mg magnesium.
You can for the most part get both from food, if you're a mind to.

As far as B12 in monthly injections... many people post on the net that they don't feel right using this regimen. So consider taking the oral methyl form daily which is more natural and easier on the body. Doctor's manuals all give the monthly regimen, for everyone. They are basically old and outdated.
Times and understanding have changed, but the references have not. It is unrealistic to expect EVERYONE to need the same treatment with a nutrient like B12.