First of all, im terribely sorry if this is doubled topic, but i didnt find any specific topic about this problem.
So here it is, im guitar student and im preparing this year for guitar competition and the problem with pain in left hand is curious, because this is happening when i play certain pieces. For example when i play relatively easy song, Bachs Prelude BWV 997, my hand (thumb, some fingers) starts to hurt really bad, lets say, nearly at the end of the song. I tried to practice it without thumb, to make the practice harded and to gain more stamina, but it didnt help that much. At the other side, I also play piece by Maximo Diego Pujol - Sonatine, and throughout all 3 parts of it, i didnt feel any problem with left hand. Thanks for all responses and sorry if bad english, not my language.