Freedom, go talk to your HR contact (the temp agency in this case) and let them know you need some accommodation with a better chair setup. I manage HR for my family company and I'd view someone like you as a comp claim waiting to happen. From an HR standpoint, they need to try to mitigate your sitting problems as best they can with a better chair, lumbar support, whatever plus scheduled breaks where you get up and move around. Be proactive --- tell someone you are struggling a bit with all the sitting and what you need to make it better. Hang in there!