28m, normal blood work 3 months ago but enlarged spleen found on ct scan and swollen lymph nodes. scheduled a follow up ct in 4 months. how concerned should i be? i did have mono roughly 13 years ago?

What was the reason for CT scan and where are the swollen lymph nodes? I do not think the CT findings are due to childhood history of mononucleosis but because your blood test results were normal so it is reasonable to wait for a follow-up scan in 3-4 months. However, if any accessible lymph node gets bigger in the mean time, node resection or biopsy should be done.