B737 Flight Attendant describes the steps taken to accommodate the lead Flight Attendant when she injures her shoulder opening the main cabin door prior to the final leg.

Narrative

Upon arriving; the 'A' Flight Attendant [(FA)] opened the L1 door and the [Customer Service Agent] did not attempt to assist; which caused the 'A' FA to twinge an old injury in her shoulder. When it came time to close she was uncomfortable with closing the door as her shoulder had started to have some pain. I; the 'B' FA; came up upon her request and closed and armed L1 and crosschecked R1. I then returned to the back of the aircraft and the 'C' FA and I armed and crosschecked our doors. The flight continued without incident; but at 10;000 FT the 'A' FA called us and told us that her shoulder pain had increased and she was not feeling able to be up in the 'A' seat solo to disarm and crosscheck doors. As a crew we decided that I would switch and sit in the 'A' seat for landing and disarm and open the doors. The 'A' FA went back and sat [in the] B [seat] where she could assist as the 'C' [FA] with the R2 and L2 doors without aggravating her shoulder. We landed without incident and deplaned just fine. Maybe make sure that all customer service agents are checked and it is made sure that they know how to assist with the opening and closing of doors.