This is not the right form of B12 for supplementation. Also, did you take another B12 test to see if this one test result was an anomaly? People's levels of nutrients do fluctuate. Did anyone look for genetic problems with folate? How did your folate and B6 come out, as these three are in balance in a healthy body? I would guess that with injections, the form of the supplement takes on less importance, as you're being force-fed the stuff, but this form would not be the one to take if you were supplementing normally -- it doesn't get absorbed or used all that well. Also, are you a vegan or vegetarian? B12 is not easy to get without eating animal food, although it is very high in fresh water algae such as spirulina, so there might be a dietary reason for this as well. Don't know about the stools -- that can happen with iron and can even happen when eating beets, but I'd ask your doctor about it just to be sure.
Vitamin B-12 at 102 is very low; you should not have ignored the low level in 2013 because it only kept going lower. My level was that low at one time and I, too, must take injections to keep my levels up.
Just because you are non-vegetarian doesn't mean you will 1) eat enough food containing B-12 and 2) absorb it once it gets to your gut... I happen to have been diagnosed with Pernicious Anemia, which when you don't absorb the B-12 through the gut. Pernicious Anemia is an autoimmune condition that you have for life.
Long-standing B-12 deficiency can cause a variety of symptoms, ranging from the fatigue you mentioned, which for me, was the worst fatigue I've ever encountered, to easy bruising. It can also cause Peripheral Neuropathy, which is tingling, burning, numbness in the hands, feet, and legs. I have moderate PN because, for many years, doctors kept telling me that my fatigue was "in my head" and there was nothing wrong with me but they didn't ever test my B-12. Once you have peripheral neuropathy, there's a good chance it will be permanent, so it's best to treat the B-12 deficiency to avoid the neuropathy in the first place... Trust me, it's miserable and there's little to be done about it...
You're correct that heavy alcohol use can deplete B-12 stores, so if you're still a drinker, it would be wise to stop or, at least, cut way back. Aside from B-12 depletion, binge drinking can be dangerous for other reasons as well.
Cyanocobalamin is the most common type of B-12 used for injections. I could see how it might affect the stools if taken by mouth, but I've never had it affect mine the whole time I've been doing my injections. Could you have ingested something else that could have colored your stools in such a way?
My lowest level was 68 pmol/L. A B12 of 0 is organ failure and death. I have autoimmune pernicious anaemia which is 100% fatal without treatment. I never had anaemia so blood cells never enlarged (masked megaloblastic anaemia). I suffered symptoms for decades unfortunately.
I take sublingual B12 (cyanocobalamin) spray daily which works well for me. This form of B12 is not retained in the tissues as long as methylcobalamin so if I stop my spray symptoms start to come back in days.
My symptoms of not enough B12 include...
Permanent nerve damage
Unpleasant body odor
Blood on pillow every morning (blood from stomach)
Bipolar mood swings
Feeling like I was going mad
Poor taste and smell
Poor immune system
Attracted a lot of mosquitoes - bites would turn into a large white lumps
Poor would healing
Premature grey hair
Enlarged red beefy tongue with crack down the middle
High pain threshold
Light sensitivity (eye pain)
Clotting issues (bled for an hour from a small nick)
Dark circles (in my case an improvement after B12)
Hair not so greasy after B12 (still have issues with oily hair though)
White spots on forearms
Had to go to the bathroom immediately (incontinence)
Folate levels decreased in the range (increased again correcting B12 deficiency)
Aversions to fish and meat (a listed symptom that is possible as I only can eat bacon, chicken, ham, mince)
Poor appetite (still have this - possibly due to gastritis from antibodies attacking the stomach)
I hope you don't have permanent nerve damage. There are many other possible symptoms of course. My sister, who also has autoimmune pernicious anaemia, had some of the same symptoms I did and other symptoms including jaundice, low blood pressure, tremors, insomnia, small pupils. She also had cervical dysplacia and had unnecessary surgery since vitamin B12 deficiency caused the irregular cells and the cells become healthy again with B12.
The B12 awareness website lists causes of B12 deficiency.
"What are the causes of vitamin B12 deficiency?
Decreased stomach acid
Autoimmune pernicious anemia
Gastrectomy, intestinal resection
Gastric bypass surgery
Celiac disease (gluten enteropathy)
Bacterial overgrowth (small bowel)
Advanced liver disease
Transcobalamin II deficiency
Inborn errors of B12 metabolism