Last few days been getting a fever around 101 in late afternoon early evening and no other symptoms besides fatigue. Why?

Virus. An early viral infection may be present and,if so, symptoms may start to localize to the respiratory or gi tract or a rash may appear. However if fevers persist or worsen,especially with shaking chills,cough,severe headache or burning on urination, see your physician as a bacterial infection requiring testing and antibiotics may be present.