Have you considered low blood sugar or low blood pressure? I have both and this is a common problem for me.
Hey. Have you ever heard of Dysautonomia - POTS?
http://www.dinet.org/pots_an_overview.htm
If it seems to match your symptoms, I would suggest seeing a cardiologist - talk to them about getting a Tilt Table Test done. Lightheadedness, fainting, etc... are classic symptoms of POTS. Do you ever feel those symptoms besides when you shower?
Just an idea, I hope you get some answers soon! Let me know if you have any questions. :)
Are you underweight? I was very underweight when I was your age - 5'5" and about 80 pounds, and I felt faint all the time. Is that what you're experiencing?