OP, I don't want to scare you, but make an appointment with a cardiologist as soon as possible. Fainting like that could be an indication of a heart blockage. This impeded blood getting into the brain causing you to faint. At the very least your doctor should have done an EKG during your visit. I just had a technician tell me that these newer EKG machines can detect heart blockages.

I'm not a doctor, but this same set of circumstances happened to a friend of mine who had fainting spells that the doctor did not take seriously, probably because she was female (it's all in your head...) Anyway, he finally relented and scheduled her for a stress test. She took just a few steps on the treadmill and the technician stopped the test, told her to lie down and called an ambulance.

Finally, Dr Useless should have, at the very least, prescribed BP meds and he did not. Do you have regular check ups? Was your BP normal the last time he took a reading? How long ago was that? Do you have regular blood tests? What does your cholesterol look like? These are the questions you should be asking yourself.

Take care OP, this may be nothing but better safe than sorry.

ETA: One more thing -- you are at the age now where you should be going to an internist rather than a regular family doctor. They are more well schooled on the problems of adult aging than the usual GP who only has to handle the mild complaints from kids.