Very weak when warm.. Why?

So whenever I get too warm when I'm in the shower,especially in the car I get very weak.. I have to get cooled off and relax before I faint! I'm 17 and 2 years ago I had a job as a recreation counsellor in the summer and we we're always outside in the warm weather unless it was raining and I couldn't do too. I got so weak and felt very sick I never went back after! I'm after fainting while getting out of the shower and I don't know why this happens to me. My doctor said it was blood pressure but never did anything..