RE: Shoulder injury
Tendonitis would actually ease with exercise and pain would be greater the day after, then training on it would cause the pain to decrease, until the day after again, when the pain would be even worse. I would say it's more likely to be a slight muscle tear, first thing to do would be to control inflammation for the next 3 days. Shoulder is already elevated, so need to worry about that, compression is difficult, ice every hour or so for 10 minutes at a time would be useful. I personally wouldn't train on it at all for a week or so, just keep it stretched. Oh, and it's likely to be the insertion of the pec major into the humerus that you have injured by the sounds of it.