god help you, when i was pregnant during winter i used to feel hot all the time, how about summer... :-(
my advise is dont wear tigth clothes something that can get air to your body. another thing keep drinking water as your skin migth dry out during pregnancy. also there is a water spray from Evian that is so refreshing you can keep it in your bag and spray it on your face and neck when ever you are out and feeling hot. you can find it in supermarkets.