I can answer this question as I experienced a similar condition.
I am in excellent shape. I noticed a pain in my rib cage on my right side. The pain then extended to my back. I could not recall doing anything out of the ordinary to pull a muscle. I was in severe pain for three days and could get no sleep. It was as if someone was stabbing me with a knife in my ribs, and then my back. My muscles were having spasms.
The usual muscle ointments did no good at all.
After three days I looked in the mirror when I was getting dressed and noticed red hive looking bumps on my ribs where the pain was, and a couple on my back where it hurt.
The flight surgeon diagnosed it as shingles.
I did find that hot showers did help relieve the pain in my back for a while, as did not wearing a blouse when possible.
I also took vitamins and drank plenty of water and fruit juices. A Greek pilot advised me to put olive oil on the area and that really seemed to help the pain.
After a week I was fine with complete rest and no medication, but it was very painful.
My best guess would be shingles. You should eventually get the red shingle hives in the painful area and then you will know for sure.
Post Edited (TyphoonEU) : 7/29/2013 7:39:02 PM (GMT-6)