Scrolling through recent posts, the "Painful Thumb" caught my eye as it's an issue I've faced, not due to strings but what @Dusty-Turtle & @ken-hulme meant about muscles. This may not fit the problem starting this forum topic, but I hope it helps others experiencing thumb pain due to muscle problems.
I've had physical (or it probably was considered occupational?) therapy on hand problems & have learned it's especially common for women. Apparently our thumbs tend to be weaker. There's even a book, Caring for the Painful Thumb - More Than a Splint, written by Jan Albrecht, an Occupational Therapist and Certified Hand Therapist. It's written so patients can understand with over 200 color illustrations, the book can be used by both patients and therapists.
I bought the book, but nowadays tend to use 2 exercises I learned. What they actually seem to do is strengthen the muscle between the thumb & index finger. Can't give you 200 illustrations, but will try to describe what I do. If more is needed, try your library to see if they can borrow the book for you. (I don't expect them to own it, but inter-library loans give you access to more than they have.) Failing that, you can find the book to buy online.
Take your hand or both hands & make a C (for the right hand it's a backward C). Have all your fingers curled in that C formed by your index finger & thumb. Lift the index finger & lower it a few times. (I tend to do it about 10 times, then switch to the second exercise.) For the second exercise, straighten your fingers out (not as straight as an L, but more of an angle -- 45 degree?); lift the index finger, press down on the index finger with your other hand, while the index finger resists the pressure; I hold for the count of 10. If both hands hurt, switch hands & do the second exercise for the second hand. If your hands are sore, you can gradually over time increase this until a time comes when they don't hurt.
I'm not a doctor or a therapist, but these have helped me when my thumb gets sore. Since, as the t-shirt says, "Old age isn't for cowards!", if anybody here or anybody you know has a recommendation of a general book of therapy exercises for various conditions, I'd love to know. I've had various bouts for various ailments with therapy studios in the past & know there's a need for a DIY book.