For a while now I have been practicing for long periods of time. Ive been practicing about six hour a day (when I can) because I have a concert coming up in a few weeks. But recently I have been getting pains in my lower back. Is there any way I can relieve the pain?
Obviously, your muscles are cramping from tension. Rather unavoidable when you sit at the keyboard for hours. While practicing, concentrate on a perfectly straight back supported by tensed abdominals. Not too tensed, of course, but contracted to a degree, since people usually forget their "abs" are the core of their body (the "powerhouse" in Pilate terms) and they let them go lax, relying instead on the lower back muscles for support. BAD idea.
When your back is tense: Get up, lie on the floor, hands to your side, head supported by a nice pillow. Then raise both legs up, but not so high that your hips come off the floor. Once raised, let your legs draw a smallish circle in the air, with the circle ending at ground level and your legs together. Raise them up, and repeat. Do this ten times. Rest. And do ten more. Make sure you are breathing during this.
This is, basically, a modified Pilates exercise guaranteed to release all tension in your lower back.
When you go back to the piano, concentrate on a straight back, strong like a tree trunk, and a back that is supported and guided by your abs.
Good luck!