Welcome to EmpowHER and thanks for your question! The temperature of your entire body rises, when you have a fever, not just your forehead. A fever is characterized by a body temperature of 100.4°F or above, as measured with a digital thermometer or temporal (forehead) scanner.
However, a high temperature can also be caused by heavy exercise, too many layers of clothing, hot baths and hot weather. Fevers are caused by illness in the body, so other symptoms are typically observed, such as fatigue, loss of appetite and rapid breathing.
I hope that answers your question! Let us know if you are curious about anything else.