Right arm numbness and left side of face numb. cat scan was clear, blood tests were good. doc is starting me on b12 injections. can it be ms or other?

You may need MRI: Cat scan may not give a good diagnosis eval. Mri of the head with contrast will be a better option.

Sorry: CAT scan is virtually meaningless in defining presence or absence of MS lesions. You may have B-12 issues, but was it proven on your blood studies or are the shots for other reasons?? Did you see a neurologist? There are reasons for arm and facial numbness and B-12 issues may not explain this.