If your arm just goes numb for awhile and goes away on it's own eventually without any other symptoms then it could just be that your blood vessels were compressed causing loss of blood flow in the arm or could be compressed or pinched nerves in the arm.
Although rare diabetes can also cause arm numbness but usually it happens in the feet first before your arm goes numb.
If no other symptoms such as symptoms of a heart attack go along with the arm numbness then you're likely fine and just resting for awhile will usually return the arm to normal without the numbness.
If the arm numbness lasts longer than a day or comes back frequently then I recommend speaking with a doctor about it to rule out anything else that could be more serious.