A couple of suggestions:

The opposite of cramping is - you guessed it, stretching. Do gentle and slow stretching of the left neck/shoulder/arm/hand on a regular basis. Even though the whole arm/shoulder area isn't involved in fingering, the blood flow to the muscles involved has to get through a couple of tight places in the joints.

Get in you're playing position and focus your attention on the left arm/hand. Is there more muscle tension there than needed to do the job? Are your joints flexed tighter than 90 degrees? Perhaps even a minor adjustment of position, or learning to relax some of the tension in the left arm/hand would help. Any chronic stress can cause inflammation and swelling that constricts nerves and blood vessels. If you're dealing with a repetitive strain injury you'll have to find a way to avoid the strain.

Work at getting at least 1 Gram of calcium in the daily diet along with 400 Units of Vitamin D (post menopausal women can do with 1.5 Grams) The best dietary sources are dairy products.

If you're taking any medications or herbs check with a pharmacist to see if they could be affecting your potassium levels. Many diuretics do. Potassium as well as calcium is involved in nerve transmission and muscle contraction.

Hope this helps.