Why did I black-out and puke?!

Please read all: I went hiking. First off I am use to extreme heats, such as 110. I've played football in this type of heat, and have been use to it all my life. I climb this certain mountain every other week. For the pass year. 3 days ago I went hiking once again, was jogging up, felt perfect. All of a sudden I felt a little sick. I sat down, puked and all of a sudden everything became black. Not only that my state of mind was a blur, I can barley remember. But I remember not knowing anything. I couldn't even think of a number to call let alone I did not have enough energy to grab the phone from my pocket to call someone. I had a sharp pain in the back of my head. ( I did not fall). A man walked by me and shook me and I regained consciousness. I barley was able to grab my water bottle, and poured it on myself which help me see again. I climbed down stuumbling all over the place. I finally was able to call a freidn to take me to the emergency room. The dr said it looked like I was just dehydrated.

My question is, could it be something else, I was having short term memory issues that say. I have never experienced something like this in my life. Also the day this occurred it was 70 DEGREES. Perfect day out. I'm use to extreme heats, I know how to stay hydrated. I only was climbing the mountain for about 5 minutes. Could the doctors have missed something? Any info on these symptoms would be great. I'm 17..