Marion has given you a lot of good advice
As to causes, anyone at any age, can become B12 deficient. However, certain people are at an elevated risk. They include the following:
Vegetarians, vegans and people eating macrobiotic diets.
People aged sixty and over
People who’ve undergone any gastric and/or intestinal surgery, including bariatric surgery for weight loss purposes (Gastric bypass).
People who regularly use proton-pump- inhibitors. H2 blockers, antacids, Metformin, and related diabetes drugs, or other medications that can interfere with B12 absorption.
People who undergo surgeries or dental procedures involving nitrous oxide, or who use the drug recreationally.
People with a history of eating disorders (anorexia or bulimia).
People with a history of alcoholism.
People with a family history of pernicious anaemia.
People diagnosed with anaemia (including iron deficiency anaemia, sickle cell anaemia and thalassaemia).
People with Crohn’s disease, irritable bowel syndrome, gluten enteropathy (celiac disease), or any other disease that cause malabsorption of nutrients.
People with autoimmune disorders (especially thyroid disorders such as Hashimoto’s thyroiditis and Grave’s disease) Type 1 diabetes, vitiligo, lupus, Addison’s disease, ulcerative colitis, infertility, acquired agammaglobulinemia, or a family history of these disorders.
Women with a history of infertility or multiple miscarriages.
Do you see yourself as any of the above "people"?
It is important to have not only your B12 but also your Folate checked as the two help your iron to make red blood cells.
I'm not a medically trained person but I have had P.A. (a form of B12 deficiency) for over 45 years.
I wish you well