Texas Hunter-
I bought a 70 lb recurve last summer and also experience a very similar and often intense pain in my left shoulder after shooting it for a brief period of time, this despite the fact that I do very heavy weight training religiously and have been for 18 years now. This includes military press and lateral raises, which I would have thought would prevent this sort of pain. I am quite certain that I developed a bursitis involving the subacromial-subdeltoid bursa, which is the bursa that separates the roatator cuff complex from the overlying deltoid muscle. Sorry about the medical jargon but I slept at a Holiday Inn last night. LOL. Seriously I am a physician, and have had some rotator cuff issues in the past. My pain was unrelated to letting off without shooting, and I bet yours is from the act of drawing back the bow too rather than letting off. Regardless though, your problem should be self limiting. Take some Ibuprofen and go easy on the shooting for a week or two. If after 2 or 3 weeks, the pain isn't gone or at least significantly better, then I would see an orthopedic surgeon to make sure you don't have a rotator cuff tear or other significant problem.
Good Luck.......been there, had that...