3 weeks of fatigue, weakness

I ve been suffering from a feeling of general weakness for 3 weeks. 2 Blood tests have ruled out most problems.  A 3rd blood test I am waiting for results.  The doctor told me that I have high blood level of B12.  I take a regular (not megadose) multi vitamin tablet only.  I don't eat much meat.  He ordered an abdominal Ultra Sound, will do it next week.
I have never felt ill for so long, I have no problem except for an enlarged prostate that gives me some urinary symptoms.   I am waiting for biopsy for the prostate though the Biopsy dr. said that prostate looked normal and she could not find abnormal areas visualy with the ultra sound.  PSA is normal for my age.

I have always been physically active and now for 3 weeks I cannot walk 20 min without feeling weak.  I am 57 but don't even look that age, my dr joked don't worry you are going to live till 100.  I would be glad to get to 80.