B12 from injections typically hangs around in the blood for about 72 hours before the tissues absorb it. If you have a test within that time period, it will be falsely elevated.

If you continue to be low even with injections, that might point to a DNA mutation in the methylation of B12 and folic acid in the body. This is common and often overlooked by doctors.
Injectable B12 is most commonly cyanocobalamin and it has to be methylated to methylcobalamin in the body to work on tissues.

I'd suggest you get a MMA test and also homocysteine. These two tests will show if B12 is actually WORKING and not just floating around. If they are elevated, you need to take the methyl form of B12 --injection from compounded pharmacies or oral 5mg a day on an empty stomach. This bypasses the DNA error and then the tissues of the body can use the B12 normally.

The DNA test is called MTHFR reductase test, and will show if you are not methylating properly. It costs about $125 now and doesn't even need a doctor's order anymore.

People here taking 5mg of methylcobalamin orally on an empty stomach often get blood work in the 1000-2000 range.
The blood levels in your body need to be high so the B12 can diffuse into your spinal cord. If you are low, not much gets there. So having very high levels will greatly enable it to move into the cord.

Severe diarrhea does come to people with very low B12. It is a ominous sign, that you are very low for a very long time.