Probably the lumbar region. Heck, you can help yourself out a lot on the cheap just by placing a small pillow right behind your lower back, assuming your chair HAS a back. And it should, if your lower back bothers you!
I won't presume to guess your age but this kind of thing comes with getting "more mature", unfortunately. When I was young I could spend hours sitting on a wooden crate and nothing would bother me. I didn't realize how lucky I was.... like they say, youth is wasted on the young! ;)
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