Dr. Mark Says:
You are describing symptoms of at least two separate problems. Sitting on a hard surface can pinch the nerves/blood vessels that go to your legs, but it should not cause nervous/circulatory changes above the legs. The tingling, dizziness, shortness of breath and racing heart you have when lying down could be symptoms of hypoxia—a deficiency in the amount of oxygen reaching the organs and tissues of the body.
An accurate diagnosis, however, can only be made with a thorough history and physical examination. I recommend that you consult your primary care physician as soon as possible.