it must be the problem of your posture. i could play forever and never feel any back pain. also, don't sit too high, which would make your hunch over keyboard and cause your back pain too.

one solution is to video tape yourself playing, and then look at that yourself to see if something is wrong with your posture, and then make adjustment yourself.

I think I found the first problem - I recently ordered a new piano bench, and the one I was using was much too high. That definitely gave me a few issues I did not have before. I just got it to the right height with the new one, and unfortunately today I pulled a muscle in my back when reaching for something! I think it is because of the bad posture/height playing for a few hours a day over the last week.

As I get better I will test it at the right height, so far it seems much better.

crash, where do you live? are there teachers of the alexander technique near you?

most conservatories offer AT and some require their students to study it. it's a great place to start with having good body mechanics for playing the piano.

it's difficult for someone to correct their posture on their own. you really need someone to observe what you are doing and help you get accustomed to what a balanced posture feels like.

I agree that it does help to have someone who knows observe what you are doing!