What could cause numbness in your arm?

Depends. A more detailed description of the "numbness" location, triggering event and related symptoms is needed to better evaluate the situation. The numbness could be from nerves getting compressed at the neck, shoulder, elbow or wrist.
Nerve. The main group of nerves for your arm is located in the side of the neck & runs toward the shoulder. Large neck muscles may be squeezing the nerves, elevated 1st rib or cervical ribs can also affect the nerve group. Osteopathic drs (d.Os.) can do manipulations to alleviate pressure on the nerve. Other causes: peripheral neuropathies (ex. In diabetics), carpal tunnel... See your dr if it persists.