Over the years I have had a variety of problems of this sort - tenosynovitis in the wrist from rowing, Achilles tendonitis from running and tendonitis in my right thumb from using a trackball while desigining my website. However I've been lucky enough to avoid serious problems from playing the guitar. The impression I've got is that (for me anyway) the problem comes if I build up an unusual activity rapidly or if I do something repeatedly that causes 'sideways' rather than 'inline' movement. In particular the problem with the trackball was because pressing one of the buttons caused sideways pressure on the second and third thumb joints.
The relevance of this is that I think that 'correct' classical guitar technique has evolved so that mucles and joints are acting as 'naturally' as possible and not being expected to make movements likely to cause damage. This raises the question of whether you have taken a really hard look at your left hand technique, not only the issue that your teacher has mentioned of 'light touch' (and not maintaining pressure when it is not needed) but also the issue of exactly how you place your thumb on the neck in relation to the fingers on the fingerboard.
I've attended summer schools with John Mills where he has video'd participants (including myself) to look at technique and I was surpised to find how sloppy my left hand technique was even though I though I was being careful to place my thumb properly. I learned a lot from doing this and I now find that I can play quite relaxed even using hard tension strings.
Whether this is relevant to your problem I don't know, but I hope you can find a solution for it