I recently got a bass with a near flat radius fretboard. Naturally it makes my fingers dig in more and my wrist to angle up, but afterwards I get a soreness in my left shoulder. We're talking 2 days total with the bass, probably about 6 hours of playing. I'm used to a Fender radius. This happened to me once before when a friend was showing me some classical guitar technique. I keep my strap at a normal level. When standing the middle of my bass is probably right at my waist. I don't have a light, nor overly aggressive attack. Am I harming myself or is this just something new for my body to adjust to? Is there any sort of stretching that would help?