I took Plan B one step three days ago after unprotected sex. Since yesterday exp. Extreme fatigue, no appetite and low grade fever (100.3) causes?

See doctor . Fever is not normal. Plan b is better if taken within 3 days (even 5 days). You should get a period, see doctor to figure out your fever, get exam and get further history.
Plan B. Potential SE’s w emergency contraception: tender breasts; changes in menstrual flow (spotting, bleeding, early or late period or spotting instead of period); loose stools; dizziness; headache; nausea/ vomiting; abdominal discomfort or feeling tired. Maybe some of these are associated with plan B or maybe you have a minor infection.