First off, relax a little bit. Pain from a cancerous tumor would have you bed ridden and basically non-functional. Second off, if it was a heart tumor, you wouldn't be able to see it; it's not going bulge through the bone like that.
If you are that worried about it, then keep a log of it to show your doctor/parents. You should keep track of the pain; when it hurts, what were you doing at the time, is it sharp, stabbing, pinching, pulsing, dull, enough to make you stop what you're doing or something that is just annoying, etc. Also see if it gets bigger. I doubt it will overnight, so take a picture where you can see it clearly once a week. Take it in the same pose and the same lighting every week so you can see if there is any real difference.
It's easy to hear something on tv or read it online or in a book and think that you have something similar. Give it time and see what happens. And quit worrying about it, it does nothing to help the situation.