When I sit on a hard surface my left arm goes numb and very painfull any ideas?

When limbs tingle and are numb, it's usually the result of some nerve damage. I'm not a doctor, so please do further research before you proceed with what I'm suggesting, but my reccomendation would be to see a chiropractor. It sounds like sitting on a hard surface triggers some misfire in the nerves along your spine causing numbness and pain in the arm.