Could be residual changes from the pneumonia -- many times the xray takes longer to completely resolve than the symptoms, even after a course of antibiotics. Sometimes, pneumonia develops in response to mild airway blockage from a mass or tumor, so if they saw something (even small or subtle), they probably just want a CT to make sure it's not something like that. Lung nodules (that are NOT cancer) are also very common, and are just followed yearly for a few years to make sure they are not growing and/or causing problems.
Apparently he saw a "shadow" and wants to be sure your lungs are okay. Sometimes it is nothing more than a shadow - sometimes there is still some infection in the lung.... Hope you get well soon.