Pain on the back of my hand between the index finger and thumb. Really hurts?

Hand pain. The area between your thumb and index finger is an uncommon area to have a problem. The space between thumb and index finger is almost all muscle with a small amount of skin and subcutaneous fat. Sometimes, with repetitive motion tasks you can get sustained spasm of that muscle, or you can get a low grade infection in the skin tissue. If the pain is over the bone, you could have arthritis or fract.