What is the cause of pain between your thumb and index finger? and how can i treat the pain?

Hand full of causes: That space contains a couple of muscles, the largest being the adductor muscle, which allows you to move your thumb toward your hand/palm. Over use of that muscle or trauma/injury can cause pain. First line treatments include ice wrapped in a towel and resting the thumb. May try Ibuprofen or naproxen. See a doctor if no better in a few days.