Back and such pain in general? Assuming there is no under lying issue its like the guys have said, muscle. Either tired muscle, muscle spasm, weak muscle, soft tissue damage.
Me, I self crack my bones and align stuff. I use a yoga ball and I also do deep stretches. I have a low back chair that comes to middle of my thighs. I have a belt neat the bottom rear legs. I stand behind the chair and loop the belt behind my ankles. I then bend over the back of the chair with my legs straight.
Make sure to do this when no one is around or looking or you may feel a swift swat to your rear end.
The reason the seat hurts you is that its not supporting your body right and putting stress on your muscles, joints, etc.