Oh my goodness! I have the exact same problem.
I had some really bad stress/anxiety related stomach issues (constant hunger, nausea, etc.) in March and went to my doctor's office to see what could be the root of the problem and try to fix it. I've had stomach issues for quite a while now so I was hoping that they would finally be able to help me out.
They ran my bloodwork and it showed that I had extremely low vitamin B-12 levels - the normal range is approximately 200 - 1400 and my bloodwork showed 110. I was completely floored because I take vitamins and my diet is for the most part really healthy.
The doctor said that this level of deficiency indicated an absorption problem and referred me to a gastroenterologist. The gastroenterologist did an endoscopy, colonoscopy, and capsule endoscopy to determine where I might be having absorption issues. All the tests came back clean. She thought I should see a hematologist to see if it's a blood problem, but thankfully my primary care physician interceded and basically said that it would be pointless and to just start treatment. Thank goodness because the hematologist would have had to draw bone marrow which I've heard is a none too pleasant experience. So in the end I had no real conclusion...
Because I can't absorb the vitamin through by digestive system supplements are useless for me. Treatment revolves around a B-12 shot given once a week for 4 weeks followed by monthly B-12 shots for the rest of my life and daily iron supplements for the anemia. Hopefully, I will be able to work something out with the doctor's office so that I can administer the shots at home, but for now it looks like monthly doctor's visits.
Honestly, I am not really feeling a difference. But then again, I just finished up my four week series and I have heard that it does take quite a while to bring the levels back up to normal.
It's a bit comforting to know that there are others out there with the same problem, but then again I feel for all of you because this is a for life kind of thing.
Good luck with your treatment!