What could lead to my left hand to be numb, tingly, and weak?

Numbness: To know for sure, you need an examination. Differential diagnosis could be cervical radiculopathy (pinched nerve in neck), brachial plexopathy, peripheral nerve entrapment such as ulnar neuropathy, carpal tunnel syndrome, or even myofascial pain with trigger points. Would suggest a pm&r evaluation for definitive diagnosis and treatment.