I say 'thing' because I don't have a definite diagnosis, and 'injury' sounds like I DID something to it, and I'm not in the least bit convinced I did. Physio (who I have only spoken to on the phone, not seen) reckons I over stretched as I told him the first time I noticed pain was doing backwards windmills as part of a gym work out, but it was a case of 'ooh, that's a bit sore' rather than anything going ping, snap, crunch, whatever.
Pain remained at that sort of level for a couple of months, kept going to the gym, and with the exception of a couple of stretches, wasn't really a problem. But I did miss my yoga class for six weeks, and when I went back to that practically everything hurt. At that point I figured I needed to see a doctor, and saw a very pleasant locum who said the words 'painful arc' which allowed me to consult Dr Google and get a few ideas.
She referred me to Physio Direct - called them and, stupidly, when asked if I wanted an appointment or to speak to someone on the phone, opted for the latter. So I did that, still no real diagnosis, unsurprisingly with hindsight, and a set of emailed exercises which I don't fully understand. I'm now booked in to actually see someone. Unfortunately it's the same chap I spoke to on the phone, and I'm not actually impressed with him - he told me swimming was 'nice gentle exercise', before thinking and amending this to 'well, maybe not front crawl'. Maybe?! Like an idiot, I tried it a couple of weeks ago and nearly passed out. Breaststroke wasn't all that much better, and fortunately I thought better of trying backstroke.
Shoulder is now getting worse all the time. Movement is becoming limited - I'm struggling to brush my hair, undo my bra, lots of things - it's my right shoulder and I am VERY right handed. The painful arc which was a little higher than the usual 60-120 degrees is now 60-180, with having my arm straight up being particular painful. I can just about still sleep on that side, but have to be very careful how I do it, I have to lie sort of on the back of the shoulder. If I sleep on my back I snore, on my front is uncomfortable (big boobs) and if I sleep ALL the time on my left I don't think that'll do my left shoulder much good! I will probably have to talk to occ health at work about the fact I can barely swipe my work pass from the car, and I'm really pissed off that I can barely exercise as 2.5 stone weight loss and hard-earned fitness are starting to melt away.
Oh, and particularly distressingly yesterday I was getting shooting pains and loss of function in the arm that nearly rendered me unable to drink my glass of wine (until I thought to switch hands). 
Seriously, I'm not filled with confidence at the thought of my physio visit. Do I really just learn how to do the exercises properly, do them, be patient? Is there more I can do, or ask for? Any experiences anyone can share that might help me see a way forward?