I have been diagnosed with mono again and have been told my spleen is enlarged. I was wanting to know is it a bad sign if my ULQ is silently painful?

Enlarged spleen. An enlarged spleen from mono can certainly be painful and would be a good reason to rest . The left upper quadrant or LUQ, is certainly where the spleen hangs out. This type of discomfort should be checked out by your doctor, however. As soon as possible. Definitely do not exercise when this is going on. .
MONO/SPLENOMEGALY. It is not unusual to see SPLENOMEGALY with mono. I would usually Recommend a course of prednisone with a taper. Also Valtrex (valacyclovir) or famvir which are anti vitals and may expedite resolution. Be careful with activities and try to avoid trauma to the abdomen. Have ultrasound rechecked to document resolution in a 6-8 wk window.