Well I've been not playing guitar much for the last 3 weeks because I get an intense pain in my back. I THOUGHT that I had been playing without any tension, but apparently not. So now my chops are terrible and I am all around extremely sad because of this. This pain is near the shoulder blade on my right side (my picking side), so for the last few days I have been trying to find a good sitting position that will yield no tension or pain...but I cannot find one! So are there any tips you guys could give me in how to correct my posture (or anything about my technique) that would get rid of this back pain?

Here are some videos for reference
Picking Hand-http://youtube.com/watch?v=IqUpGHCRIPA
Overall Posture-http://youtube.com/watch?v=dn12eq4rI9I

Any help is GREATLY appreciated, I start giving lessons in about a week and I don't feel like having terrible technique.