Why do I get so sick after being out in the heat for short periods of time?

In the 85+ degree weather lately I have been getting really sick after being outside for very short periods of time; 30 minutes or less. When it cools down to 75-80 degree weather i am fine. When I am out side and this happens to me I am not doing any strenuous activity, if any thing just walking very short distances. Often all ui am doing is sitting because I do not want this to happen, but it does any way. :( What can I do and can it be prevented without having to stay indoors all day.