After about 20 mintues or so of playing, my back starts to hurt. Most times, to the point where I have to stop playing. I think it might be because of the chair I sit in when I play. It's back angles back a little, so I have to hunch over a bit so the guitar is vertical, otherwise if i sit with my back against the chair, my guitar is kind of angled towards me, kind of like its lying flat, but not that extreme. Obviously, this makes it hard to play. So, should I get a different chair, or am positioning myself oddly?
Listen to CKY.
stand up, see if that fixes things.
"There he goes. One of God's own prototypes. Some kind of high powered mutant never even considered for mass production. Too weird to live, and too rare to die."-Duke
if you wanna sit, put a cushion or something behind you to straighten you out

put tiger balm on your back where it hurts before you play... um id say positioning yourself wrong but i dunno
get a new chair
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get a strap and stand up...if it stops hurting then get a new chair, personally i dont use the back of my chair when i play anyways
people with large sig's are clearly compensating for something.
Yeah, a new chair, or standing fill fix your problem; either that or seeing a chiropractor.

When you stand, remember to have your strap adjusted well enough so that it doesn't sag down and pull your back out of good posture. :P