In reply to GraMc:
I pop in from time to time on here to say (as a physio) there is no bad posture, there is no good posture. If you stand still like a soldier at attention (good posture) for 8 hours a day, 5 days a week things will start to hurt. The problem isn't the position, it's the time spent in the position.
Regular movement breaks are all that is required to stop things becoming a problem, even if your monitor isn't at eye level.
Once you have problems though then yes i'd (obviously) recommend you see a physio.
There's been some research done recently on where this pervasive idea of good/bad posture has come from. Believe it or not it can be traced back to ancient times, religion and then the good old Victorians. This was to do with your appearance reflecting your inner morals, not muscle health. But then in modern times we stick an entire business model on the top of that (physio's, chiro's, health and safety at work etc) and it's no surprise it's stuck around as an idea. We're taught it.
I hope you get a solution to your issue quickly.