Why would my B12 stop absorbing and then ten years later start absorbing again?

Over the past 13 years, I had been very ill. I had a full hysterectomy in 2008. Before that the doctors said that there was no sign of menopause or peri-menopause. Afterwards, I was still very ill. Then a doctor did a full panel on me and found that my b12 did not register. I started on b12 shots and immediately got better. Now, years later, my b12 is suddenly reading in the middle numbers...Why?