I know this must be a really frightening time for you, but the fact they have referred you to a paediatrician at the hospital is great; it means they're acting fast in making sure everything is okay.

First and foremost, bowel or colon cancer at the age of 15 is incredibly rare. I can't deny that your combination of symptoms are definitely a reason to have further investigation, but they could just as easily be something else entirely unrelated to cancer (and, honestly, the statistics are for it not being cancer!) 

Red blood in your stool could, for example, be an anal fissure. Red blood and pain in your left side (where your colon descends) could be a polyp which are mostly non-cancerous and quickly removed. Or, perhaps even some IBS. 

Anaemia can be a symptom of bowel cancer as a result of bleeding in the gastrointestinal tract, but I'd expect you to have described having incredibly dark/black stools quite consistently, rather than just irregular blood here and there. For me, it doesn't sound like you're losing enough blood for the two to be connected. :)  I'm unsure your gender, but anaemia can be quite common in females, especially if you're in your first couple of years of starting your cycle or have particularly heavy periods.

I know it's difficult and worrying is inevitable, but try to keep your mind off of it. -- Google searches will make anyone more anxious. Medicine is infinitely complex, and there are so many different factors to take into account that symptoms lists on Medscape or WebMD can't consider for you. Though, I'm glad it's brought you here, and I hope anything I've said may have helped calm your nerves.

The worst part is not knowing, but you'll be in good hands. You got this!