What would cause me to have a low-grade fever in the evening for only about 5 hours??

Usually puzzling: What causes low grade fevers is not usually easily discovered. I would first consider if there's any particular activity or food usually done around that time as a first possibility however the list of possibilities is long. Basically if this goes away you can forget you ever had it, if not you need to see a doctor to start evaluation which you made take a long time