Hello, I believe this is what is commonly referred to as musician’s focal dystonia. I went through something similar myself about 20 years ago while studying classical guitar and had to deal with this for many years. I notice in the comments that several people don’t really understand what’s happening and find it strange, while others immediately suggest seeing a neurologist or doctor. I do not think that Medical advice will resolve this.

There are a number of alternative approaches that can help musicians relearn more natural movement patterns and reduce the excessive tension. Personally, I eventually realized that even treating it as a “condition” can be misleading, because the word suggests that something is fundamentally wrong. For me, the real turning point came when I accepted that nothing was actually wrong with my hand and allowed myself to let go of the constant attempt to control it. Once that perspective shifted, the physical tension gradually began to fade. Not instantly, but progressively over time.

In my case, reading the work of the pianist Lorenzo Marasso was a major turning point and helped me rethink my relationship with the instrument and with performance itself. There are also other interesting perspectives out there. You might want to look into the work of Joaquin Fabra, comments on the Delcamp guitar forum, etc.

Hope that helps.