Confused as to why Scatica made me go unconsciouss and why I have Scatica?

Here is some background for my question:
I fainted friday night. I had a minor headache all-day Saturday. Felt really worse and that I might faint late Sunday afternoon so I was taken so to the ER.  Never ended up fainting, but was put on an IV and had blood tests done. All blood tests came back normal, so I was released and told that it would be a watch and wait thing.  But then I woke up in the middle of the night last night with a bad headache and I could hear a strong pulse in my head. Took aspirin.
(If there are any further details about my situation that you need to know to ***** my situation, feel free to ask)

Today, the left side of my head hurts badly w/faint feeling from time to time so I made an appointment.
I visited a primary care specialist, and was told that I fainted Friday night in direct result of the sciatic nerve pain that I had experienced in my right butt-cheek seconds before I collapsed.  I asked why I experienced the sciatic nerve pain, and he said something about having back pain, and I told him "I have never had back problem in my life" so he said if I feel the nerve in my butt again then I should consider physical therapy. Then, he said to continue to take Tylenol for my headache and I was on my way.

After doing some internet-research on sciatica, it seems like it has a lot to do with hurting your back, or having a herinated disk, pinching a nerve, etc. I have never had any serious problems with my back, and I wasnt doing any activity at the time or even that day that could have hurt my back.  And I have no soreness from the sciatica, while it seems that typically you would. And, doesn't it seem more common that an older person would have scatica? I am 21-year old female, healthy weight, and I excersice a few times a week, and have never fainted before, not even close. And last, I have never read anything about anyone fainting from it (the actual scatica, not the pain), although maybe people sometimes do.  

So basically, my questions are:
How did I get scatica?
Why did *I* faint from it?