Large red spot under skin on forehead when nervous or hot. Forms as the same shape each time and also appears when blood rushes to forehead. Any idea?

Nevus flammeus. What you describe is a vascular birthmark. It is due to a congenital (since birth) malformation of capillaries (small blood vessels) under the skin. These usually fade or become less obvious with age but can persist. The redness becomes more obvious with increase blood flow to the area. Treatment is only needed for cosmetic reasons. The pulse dye or vascular laser is the preferred way to treat.
Hemangioma. Hemangiomas are abnormal collections of blood vessels. They can form in virtually any part of the body. Usually they are harmless. Visit your doctor to verify the cause. If it is a hemangioma and it is cosmetically unsatisfactory, then there are ways to deal with this. Usually a plastic surgeon or a dermatologist is your best bet.