Last week's long swim within a long week seems to have taken its toll. I have a dull ache around my scapula and occasional twinges from an area I imagine the brachialis to be. I don't believe either of these symptoms indicate even referred pain from an impingement, which is a good thing.
This week I have a stack of tasks after work making it unlikely I'd find time for a swim, so that is a useful opportunity to try to rest it and hope for recovery. I've a massage booked for Thursday which will be helpful if the pain merely indicates tightness in the back and shoulders.
Here's a useful introductory article on swimmers' shoulders.