Here is what I went through, maybe it will help answer your questions:
A couple of years ago I had a cyst on my spleen. I was active in sports but didn't know of any time when trauma may have caused the injury. The pain was especially felt in my left shoulder but began hurting in the upper left side (front and back) of my abdomen. I initially thought the pain in my shoulder was a badly pinched nerve or my back being out of alignment, but it was actually the spleen sending pain signals through the nerve that runs from the diaphragm up to the shoulder (known as referred pain). It was painful to lean back in a chair, to lay down, to breathe deeply, and sometimes movements in general (like the vibrations caused from walking).
The pain and discomfort got to the point that I eventually went to the emergency room. They did a CAT scan and found a large cyst on my spleen. Soon after they did a peculation to drain the cyst and see if that resolved it. They drained a liter of fluid out of the cyst! It was mostly comprised of old blood (no cancer or anything). Unfortunately, the cyst came back and they had to result to surgery. Initially my doctor thought he was going to have to remove the spleen, but he decided to call around for second opinions (thank you doc!). He found an alternative solution and ended up doing a laparoscopic procedure that removed the cyst and then he burned the surface of the spleen to try and prevent it from coming back. It worked. I still have my spleen today.