Vitamin B12 deficiency and assorted gastric problems

I've been diagnosed as having a vitamin B12 deficiency.  My B12 level is 152.  I received my first B12 injection just over a week ago.  I'd felt fatigued for a few months prior to my diagnosis.  At the moment I feel even more fatigued.  I'm also experiencing some pain in my chest, back and arms.  It can be mildly uncomfortably, tingling, or occasionally a stabbing pain.  I was wondering:
a. if the pain is normally associated with B12 deficiency
b. how long after starting injections does one usually feel the benefits.

I've also been diagnosed with a mild hiatal hernia, and gastric erosion on the greater curve (biopsies from endoscopy are pending).  I was checked for any cardiac problems (as I was initially admitted with chest pains) and cardiac tests revealed benign premature ventricular contractions but nothing more serious than that.  I was wondering if the chest/back pain could due to the gastric erosion.

The pain can be distressing, and I'm given to being anxious about it.  I'm a 30 year old male.  I don't smoke, and don't drink.  I'm not very active in the exercise stakes, but am very skinny - I weigh 126 pounds, and am 5'9.  My cholesterol levels was 4.4

Any help or advice or words of wisdom would be greatly appreciated.

[Manysymptons]

I too have a low b-12 count, 168, and my doctor has me on two shots a week for 5 weeks, they will then re-test my b-12 count.  My doctor told me that it could take 2-3 months to raise it to a decent level, she wants me at 500-600, and I have read that alot of people with low counts continue to get shots or take sublinqual supplements for the rest of their lives.  Please let me know how long your shots take to start taking effect.    GOOD LUCK!