What Causes Abdominal Pain And An Enlarged Spleen?

My daughter is 19, had mono for about 4 weeks. She is in college 3.5 hrs away. This past week she had pain in her left side - enlarged spleen. Finally got her to go to health services. They called said she was exquisitely tender and felt she had an enlarged spleen. They wanted her to go to the ED. She did. This was yesterday. CT and lab work was unremarkable . Though she had a UTI. I specifically asked the indication on the CT - left side pain. I cannot ascertain if there were measurements. They gave her Rx for macro bid and norco. She still has pain. She refers to it as a side stitch like when you run hard. It hurts to lie on her left, cough, palpating. She says it feels deep in the front but superficial on the side and towards back. What am I missing? What am I not thinking of that could be her issue? She does not typically have pain or is a complainer.