Recently I have had a couple problems with my fretting hand (left hand). Starting last Friday my wrist became sore and stiff for no reason ad didn't stop for about two days. Tuesday to today Ive had pins and needles and itchiness in my knuckles on my fretting hand. Is it possible for me to be devoloping arthritis? Im 18 and just joined a band so this would be a huge bummer.
The best advice is to first, see a doctor and get your problem diagnosed. It may not be bass related.
When you get the all clear to play your bass, from the doc, you could check your technique. The links below should help. Make sure you are not gripping the neck too hard, or pressing down too hard on the strings when you fret. Keep the wrist as straight as possible. Do you wear the bass very low ? If so, try wearing it with the main body of the bass at around belly button level. Make sure to warm up before a practice session or gig. Do gentle stretches of the hands and wrists.
Ya, I suspect death grip on the neck causing it. Probably getting a little too wound up when you play. Relax a little more, learn safer left hand techniques, and no death grip.