I have fibromyalgia. I believe it comes on slowly. So slowly that you don't even know you have it until all of a sudden you have all kinds of symptoms all at once. Have you had any other type of ongoing problems? Something that perhaps you never gave a second thought to and now it has all compounded? Yours doesn't really sound like fibro to me. Go on line and look up the symptoms for fibro. There are lots of good sites out there.
This doesn't really sound much like fibromyalgia pain - that pain is typically widespread and not just in one area (although certain areas can feel worse at times than other areas). It also usually affects both sides of the body pretty equally.
It sounds more like maybe a shoulder injury (have you fallen or stretched your shoulder at all? heavy lifting?) I could also be a pleuritic type pain (inflammation of the area around the lungs). You may want to try an over the counter anti-inflammatory type medication such as ibuprofen to see if it helps the pain at all.
If it continues, I would most definitely get it checked into further - I dont' want to alarm you, but just because the testing done at the hospital didn't show any problems with your heart, it's still possible that there is something going on there.
the anti-anxiety medications may have helped partially because, at least for me, when I'm in severe pain (quite often), it makes me tense up and get a bit anxious, which only makes the pain worse. If you can try to relax during these times (I know, almost impossible), it may lower the pain level just a tad.
Best of luck.
it could well be a frozon shoulder that can be painful it doesn`t not sound like fibro, or you could of laid awkward on it that sometimes takes days to go. have you got any other condions ?