I have pain in my left middle back after eating what could cause this, thanks?

Suspicious. Pancreatitis -inflammation of the pancreas- is clsssically described as upper abdomen pain radiating to the back. May also be gallbladder or liver related. This whole system is biliary system. Blockage of blood supply to GI tract or ulcer possible. Best to be seen soon as some of these can be very bad. Workup may involve blood tests, ct, ultrasound, and more. If vomiting or jaundice, go to er now!
Could be an ulcer. Occasionally a peptic ulcer in the back of the lower portion of stocmach or upper intestine known as duodenum can cause such pain. Also, gallstone can cause pain in back, but more commonly on the right side. You should consult your doc to find out the true cause of your pain...