What Causes Dull And Fluctuating Pain Under The Left Side Of Ribs?

Good afternoon,I have a sporadic but recurring pain in my left side just under my ribs - feels as if it occurs about halfway between the sternum and my side. Dull, nausea and fluctuating pain.No obvious change in diet or bowel movements. Tired.51 yr old male - generally fit and well - possible coaeliac - awaiting confirmation.THANKS!