Itchy red spots appeared overnight in my upper chest. They range from half to a quarter of an inch and have no visible raised head. Any clue?

Sudden itchy rashes. on the torso usually have allergy or hypersensitivity causes. I would monitor and apply an antihistamine cream or ointment and monitor for duration, relief or change of symptoms.
Dermatology consult. unable to answer without a photo and examination. Get a dermatology evaluation.