I think it just happens, especially with people prone to anxiety. I get it all the time. I can be totally relaxed or even asleep and it will happen. My temp will even go up and sometimes I will start to sweat, but i am never actually sick. I usually sip on a coke or on water. That usually helps for some reason. If it's not anxiety it's my blood sugar. When my sugar is low I get really sick feeling and faint.