You need to look at the cause of the shoulder pain, not mask it by having an injection. I suggest you switch your PC off and unplug it and spend a week not using it! I have had similar problems and when I had a week off work and went on holiday (away from computers) it gor much better. It sounds like repetitive strain, and by continuing to use the PC you are making the problem worse.
Do you have a mobile phone? If so, get your provider to enable internet on your phone so you can still check your email, but won't be sat in the position you are now. Also maybe find other things to do? Computers totally eat our time, and there are lots of other fun things to be doing outside etc.