I have been playing guitar along time. On and off heavy strings. I do use my thumb alot with some chords. I have developed a lump the size of a marble right below my thumb on my wrist. It feels like muscle because it's not hard until I flex it. Is that weard or what?? Could it be damage?

I would go doc's mate. it may be cyst or something, best get it checked

I used to have something like that on my hand - but it went away.

I went to the doctor - he said he could cut it out - but I never did it.

I have this rule. If any strange lump, bruise, feeling, or pain appears and last more than a few days I go to the doctor.

i forget what you call it, but it has to do with the way you play the guitar. i had that, and it went away when i changed my left hand style.

Right below your thumb on your wrist sounds like muscle development, but if you feel it´s not it´s deffinately a 'thick skin', and if it hurts go for a doc man.