My left arm & hand have been tingling for months. No idea why. I'm 25. No heart attack. Could I be sleeping on it strangely?

Yes, position impt. Assuming it is intermittent and is most nticable in the am upon arising, sleep position is a real possibility. Suggest using pillows to force you to sleep with er on the back or oppositie side. Play detective related to possible causes which are mechanical. Truck drivers get it with the left arm propped on a ledge or arm rest as an example. If it persists and exam would be reasonable.
Carpal tunnel. Carpal tunnel syndrome is characterized by hand pain and recurrent numbness or "pins and needle" sensations in the thumb, index, middle finger and the lateral aspect of the ring finger. The symptoms may be triggered by use (driving, keyboard use). Your physician may order a nerve conduction study to confirm. Wearing a cock- up splint is very effective. Surgery reserved for severe cases.