People with the Epstein Barr virus sometimes feel pain in their upper left abdomen. This is because these individuals have developed an enlarged spleen. People with mononucleosis often develop an enlarged spleen within about three weeks after they become infected. A sudden sharp, pain in your upper abdomen is a sign that your spleen could be ruptured and you should seek emergency medical care immediately.
Pain in the Left Side of the Body with EBV – Enlarged Spleen