1. Stop playing. See a health professional.
2. Stretch before and after you play. From Gary Karr: "The most important stretches that should be done both before playing and after playing ALL address the shoulder. In my experience as a performer and teacher, I have come to realize that many of our aches and pains come from an inactive, stiff shoulder. It affects the blood flow to our fingers and, used improperly, can cause problems to our elbow, wrist and finger joints. Therefore, anything you do to limber up both shoulders is a good thing. Here are a few suggestions: Bring your arm in front of you parallel with the floor, bend at the elbow and bring your hand over your shoulder. With the other hand or arm, pull the hand over the shoulder as far back as it will go and hold it there for the stretch. Another good one is to raise your arm toward the ceiling, bend the elbow and let the hand fall behind your back. With the other hand, pull your elbow back as far as it will go so that the hand can move further down the spine and hold for a good stretch. My favorite, which I do many times every day is to clasp both hands with straight arms behind the back and pull upward for a long stretch. You can also bend over (knees slightly bent) and bring the clasped hands and straight arms over your head, and hold for the stretch. I'm sure you'll find these a good start."
3. Arch your fingers. Hang from the fingerboard like a climber would from a rocky wall. The weight of your arm and back are entirely sufficient.
4. Try rotating your hand and forearm when putting your second, third and fourth fingers down. You are not generally pressing notes with your finger, but letting the rotation and weight of your hand attack the note. Velocity decreases the amount of finger strength that you have to use.
5. Don't maintain stetched positions.
6. Relax. Breathe. Use vibrato and keep the hand loose.
7. Have someone check your setup to make sure that it isn't too high and that the strings aren't too stiff.
I always thought the term "fingering" was a bit of a misnomer. So much of what goes on is in the larger muscle groups in the arm and even the back.