What Causes Pain In Abdomen Radiating To The Back With Constipation?

Most times when I eat, I am experiencing pain in my upper abdomen, dead centre in between my ribs. The pain then spreads to the sides and sometimes as though to my back. The initial pain is a burning sensation, so I at first put this down to indigestion. However, several weeks down the line it is now getting irritating and very uncomfortable and to be honest quite worrying. I seem to bloat very easily, have had a major loss of appetite at times, experience nausea and the feeling to go the toilet a lot, have quite irregular bowel movements and on 3 occasions have passed a small amount of blood.