Very many things....while heat and dehydration may contribute to your symptoms it is advisable to be evaluated for other causes including infection or serious systemic allergic disorders
sounds like sun poisoning or heatstroke. antihistamines wont do much. drink plenty of fluids, take a cold shower even though you're shivering, and stay out of the sun and heat. if it gets worse or doesn't go away in a day go to your doctor immediately sun poisoning can be very dangerous.