When you start B12, because of being low, the B12 is prioritized by some body mechanism to be sent to critical areas..one of which is the bone marrow.

Here it gets everything going to replace the blood cells the marrow makes. Each cell gets some B12 also, along with increased use of potassium.

If you are low in potassium to start with, or marginal, you can get extreme fatigue and weak feelings. So eating high potassium foods may help this. You can get tested as well, but potassium often shows normal or high when it is not. This is due to the lab mishandling the sample ...which involves bursting the cells and dumping the potassium into the serum. If you show a real LOW that is probably accurate. But some normals and highs may be factiticious (falsely elevated).

This fatigue will go away as you use the B12 to normalize the bone marrow cell factory.

It takes time to fix low B12. The liver, will grab some of each dose for storage, and some will go into the spinal fluid and to the brain. These 3 sites have a high affinity once the B12 therapy is started. It took you months and years to get where you are now, so be patient as your body restores itself and stores some B12 for the future.

Some people find that weak muscles may also respond to some Acetyl Carnitine. This improves mitochondrial functions which we feel as "energy" and strength. Start low at 500mg a day and you may increase if you think you need to, by 500mg a week up to 2000mg a day in divided doses.