As the evening wore on, the discomfort increased and I couldn't raise my arm laterally without a significant amount of pain. After two restless nights of sleep and a bit of panic (I am scheduled to start my race season in 9 days), the wonderful David McHenry at Therapeutic Associates did an evaluation this morning.
His conclusion is a strained supraspinatus muscle-- one of the rotator cuff muscles. Nothing torn and nothing that ice and rest won't alleviate. No swimming, cage fighting or chicken-dancing for a few days. So the Hagg Lake 4K swim is out for Sunday, but hopefully I will still be able to race at Granite Man.
I am now looking for a dry-fit bubble suit to run in.