Shafty, I'm glad that your posting problems were fixed and that you can now post. It sounds like your problem might be a strain or possibly some impingement (or maybe a combination of both).
I'd highly recommend that you start working on improving your posture ... check out the
Neanderthal No More series of articles at T-Nation. There, Eric and Mike give you some great advice on how to fix your posture (there are 4 articles total).
Then take a look at
Push Ups, Face Pulls, and Shrugs where Bill and Mike give you some good ideas for shoulder health ...
In the meantime, if the shoulder is sore, rest it and ice it.
What does your current program look like? I'm curious to know if your pushes and pulls are balanced.