Fever with no other symptoms

I woke up yesterday around 3 in the morning to check on Tyler and noticed that he was hot from head down to his arms and legs and thought he might have a fever but when he woke up in the morning he was happy even though he still had the fever. He was at 101 F when we checked his temperature and gave him one dose of medicine and plenty of fluids. We were wondering what caused his fever since he has no other symptoms and no changes in his behavior. Today he still have slight fever but he doesn't complain about it and he slept good through the night. We will observe how it goes throughout the day and hope that his fever will go away soon.