My left wrist is very painful at the base of my thumb and sometimes spreads to my whole wrist and up my forearm a little. Why?

Arthritis or break. There are 3 bones in your thumb: two in the digit and one in the hand. Pain where the digit meets the hand could be in the joint (arthritis) or one of the thumb's tendons. Pain at the point where the bone in the hand meets the wrist could be a wrist fracture. These are sometimes missed on initial x-rays after an injury. Either way, you need an x-ray of the thumb, hand & wrist.