I've been playing seriouslly (aka: actually practicing) for about a year now and went though this issue not to long ago. I found out that I was clutching the bass to tightly with the left hand when I started getting shooting pains in my left hand/wrist..
After taking a break, I did three things. 1) Lowered my action. 2) Changed my bass/body/strap relationship to ease the angles. 3) Focused on my evil pressure thumb until I rooted it out.
It's been about 6 months since and I'm pain free. Woo
