Well are you? I’ve watched players practising a new chord or negotiating a tricky passage and concentrating so much on getting their fingers in the right place they haven’t noticed their body folding into wild contortions, as if it is their arms and legs that will make the difference rather than their left and right hand. Not surprisingly strain can occur as a result of poor posture. For both guitarists and mandolin players the traditional positions for playing used to be with one leg crossed over the other with the guitar or mandolin supported on the upper thigh. Although this feels natural because it places the instrument in a useful playing position it is in fact forcing the spine to curve and the pelvic girdle to twist. As a double bass player I used to lean into my bass and had painful back problems. When I finally got to see an Osteopath he took one look at my spine and asked “Are you a bass player? You have a classic bass players curve distortion in the spine”. I changed my playing positions leaning the bass into my body so I could stand straight and shift my weight easily from foot to foot and my problems disappeared.
So finding a comfortable, and skeleton friendly playing position, is very important. Not only for health but also for your playing. Both feet on the ground is a good place to start. If you need to raise your lap then use a foot stool for both feet so both are raised. Check your shoulders are level and you are sitting straight and relaxed. Check for tension and practice releasing it in your arms, neck and shoulders. I experienced quite a lot of muscle pain in my thumb and back of hand from switching between bass and mandolin until I consulted a Feldenkrais practitioner who spotted a problem in my shoulders.
One good tip I was given was to practice in front of a mirror so you can see how you are sitting as you play and also where you are holding tension. Change your posture until you look at ease and balanced and notice how much easier your playing becomes when you do this.
If you are experiencing pain or discomfort as a result of playing then first of all STOP before any permanent damage occurs and seek some professional advice. A good place to start is with an Alexander Technique teacher.
Look after yourself.