You know, I'm the same way. And I've been told it's probably due to my thyroid being low. And I'm on treatment for that, so when it kicks in, we'll see. But I also think maybe it has something to do with tending to have low blood pressure- at least in my case. I tend to have rather low BP and hot showers and hot summer days make me feel like I'm going to black out. Been this way since I was a teen as well and now I'm 31.
For years I attributed it to having had heat stroke when I was a camp counselor at a summer camp... because I've been this way ever since. But whatever it is I think it's pretty benign, though annoying.