I Worked In The Heat Yesterday For 3 Hours Standing

I worked in the heat yesterday for 3 hours standing in the sun without any shade. I did drink a 28oz bottle of Gatorade and a 2 liter of water. I sweated profusely and started to get a little disoriented. I developed a sinus headache, stuffy nose, coughing, and achy eyes. (what is causing this to actually happen?)