I am suffering from hand weakness... I have irritable bowel syndrome and B12 deficiency... Is that the cause of my severe hand weakness. I'm 19 years?

Surprising issues. Most cases of b-12 deficiency occur in older age group. Who made this diagnosis? Need to check b-12 and methyl melonic acid levels. If hand weakness on one or both sides, could be due to nerve compression at wrist or elbow, and may be carpal tunnel.. See a well versed internist and address all of this. May be see a neurologist later.
See doc. Severe hand weakness without mention of other symptoms of peripheral nerve irritation / dysfunction is concerning. You should probably see your doc to have this evaluated. Could be a problem anywhere along the course of the nerve all the way from the brain down to the wrist.