Hey there, 

At your age my children's dad had similar. He was very unwell with those symptoms for a while. Eventually it turned out that the blood in his stools and sickness was food poisoning which lingered for ages and it ran into glandular fever which explained the tiredness and headaches etc.... He was a fighting fit fireman and it completely knocked him off his feet for months. It's brilliant that you already booked an appointment. Make sure you tell the doctor of your worries, don't feel daft for worrying and googling because we all do it! I myself had a few scares around your age too, I had a lump under my arm, my white cells went down to 0.3 and they're meant to be between 2 and 10, a few things happened and I was absolutely petrified but each time I was ok and I'm still here! I feel for you because the anxiety that sets in is unbearable when youre worried but look up relaxation techniques, try a few because as simple as it sounds even deep breathing will calm you. Chatting here really helps too because everyone is so supportive. 

I hope that helps a bit! Other's will be along soon to help too, I'm sure. Take care. 

Xxxxxx