Okay, the painful arc is typical of impingement so you're probably on the right track with the guess. Most people think something is impingement when it's not.
Do the LYTPs, work on more scapular depression and retraction, to help retrain the scapular muscles.
Loosen up the external rotators with some tennis balling or stretching + t-spine mobility with foam roller
anti-inflams if it helps
Basically the above + let it calm down for about 4-5 days and then re-integrate some range of motion or light barbell work if it doesn't hurt... preferably isolation exercises to start off with