What causes very bad pain when you bend your finger?

Arthritis or joint inflammation is one possibility. A minor injury may have dislocated the finger, or stretched a tendon. Muscle pain can also be a sign of circulation problems, and any prolonged discoloration should be brought to the attention of a medical professional.

One form of painful joints associated with aging is called "trigger finger" and often includes the locking or snapping of the joint (see related link).