The low potassium occurs only in those with severe anemia.
The B12 jumpstarts the bone marrow to make new red cells and they rob the body of potassium for a brief time. This stops when the red cells reach a normal level. Severe anemia is one of the last signs of low B12. Neurological symptoms and peripheral nerve damage often precede anemia.

There are types of kidney tubular acidosis that waste potassium thru the urine. There are also types of kidney disease that cause potassium to accumulate. Also the elderly can bleed from the colon if they have cancer there. I know an elderly woman who started bleeding internally from cancer and became very anemic and weak. It was the first sign of her colon cancer.

Low potassium can occur when people stop eating properly.
It is best to get potassium from high potassium foods. Even V8 juice is high in potassium.

The elderly are prone to all sorts of imbalances..and guessing is just not safe.

If and when you seek a doctor stop the B12 for at least 5 days before, so the test will be more accurate.

I still think seeing a doctor at this point is the best solution.