What can be the reason for a 101.5 fever in a 22 months old? No obvious symptoms...?

Last night my 22 months old had a 101.5 fever. I gave him Tylenol and fever went down for 5 hrs. Today he had a lower grade fever, highest 101 and I keep giving him Tylenol every 6 hrs. Everything else seems ok with him, he is not complaining about any pain, he is not congested, he is not teething, he is not pulling his ears. He is playing and laughing - not as much as usual though...he gets tired more easily or wants to be hold from time to time (which I really enjoy) Do you have an idea what can be the cause? The only new thing I noticed is he is sneezing but we had really windy weather these days. Can allergies cause fever but no other symptoms...? Thanks