Vampyre, I am so sorry you had to see such a thing. You body works hard to keep the truama "under wraps" so to speak. It pushes the feelings deep so you can continue to try to live your live as normally as possible. At some point, the energy it takes to hold the feelings back becomes too much for the body to handle. Then add the stress of being a teenager ( and there is alot of stress there), and then just going through the daily stressors, you body becomes less able to cope with the stress. It malfunctions and you begin to have symptoms: depression, nightmares, stomachahes, headaches, anxiety, fear, agitation, daydreaming, etc.....
This is something that needs some professional help and support as you grieve the loss of your friend, the loss of your feeling of safety, etc.
Someone trained in trauma therapy would be the thing to help you. You are young adn need alot of support and we are all here for you...many safe hugs...if you want them..