I am on light duty for a shoulder injury. Now my back is hurting. Do I report this? And to whom?

I am a bedside registered nurse on a busy cardiac floor in an acute care hospital. I sustained shoulder injury 2014. Long story short, Oct 2015 I had a flare up and was unable to return to full duty. My employer gave me a hard time about what light duty jobs were available but after some discussion, I am now on desk duty. After sitting for 12 hour shifts, my back is now taking a toll. Very painful at times that I can't sit still. Can I report this to my WC doctor? Or do I report to my employee health as a new injury?
As when I returned to work, employee health had me sign a Return to work agreement, including mandatory lunch and breaks and no overtime. What happens with high census and no one is available to relief me? They do pay me overtime for working straight but my body really feels the pain after the 12hr shift. Who is at fault? Can I sue them for violating their own contract?