Hello gildor64, back pain is something that many classical guitarists suffer from and I suggest you look in the following section for many topics covering this ailment.
Often, footstools can cause problems, as they did for me. But iif you are self taught, it is a good idea to have sessions with a reputable guitar tutor to make sure you are, in fact, playing with a good posture. I guess you played cello with both feet flat on the floor and now you are using a footstool for the guitar.
For many people a footstool is fine and produces no problems, but for others, guitar supports are more beneficial. These enable you to play with both your feet flat on the floor.
Please visit the following link to introduce yourself as you are new here.
It might also help you to look here too, the new member's page.
Finally, I do suggest you see a specialist to check out your back problems.
I shall move this thread to the Aches and Pains forum.
All the good music has already been written by people with wigs and stuff.