I'm glad to see I'm not alone. Endo is a weird illness, isn't it? I reckon that there are a whole load of symptoms that us women with Endo suffer, that doctors aren't even aware of, or would not even relate to Endo!
Anyway, I can sort of understand where you are coming from with this back pain issue. I have back pain with my Endo. It is mostly lower back, and predominantly radiates out from the middle of my back, generally slightly worse on one side or the other (often my right side). It can be aggravated by walking, stretching, housework, things like that. It tends to be there all the time, at best as a dull, but slightly nauseating aching. At worst it is a cramping very sharp pain. I joke sometimes that it feels like I've been kicked by a horse!
However, I do also get pain higher up, again mostly on my right hand side. It is sort of at rib level, and is sharp and very uncomfortable pain. I have noted that it is worst around the time of my periods, and also worst when I breathe in. My back and right side feel tight (like they might if you have bruised ribs, or if you have been coughing too much). I would describe the pain as starting on a level just above my waist, and ending just below my armpit. Worst at the back and side, no pain at the front.
I have no idea if this is the same pain that you describe... but it does sound similar. My pain has also only been present, as far as I can recall, since I've had Endo. I just wondered if maybe you would like to reply to this post, and let me know a little more. I've heard that some people can get Endo that spreads to their lungs; however, my Doctors refuse to even entertain the idea of this, so I cannot discuss it with them. It would be great to share experiences with somebody who is going through something similar.