I found a hard knot/bump on my chest next to my sternum under neath my collar bone.?

Ok, so about 2 days ago, I found a hard bump, or knot it feels like underneath my collar bone(Clavicle), and on the right side of my sternum. Right now it feels really hard, just like a knot would be. I'm really scared, cause im only 14 years old female, and I really don't want to have cancer. I'm thinking its like Lymphoma, And I'm a huge hypocrondiac, and over the summer like 6 months ago, I thought I accualy had lymphoma, from something in my throat, but my doctor ured me I was okay, and I didn't have cancer, now im just freaking out, I don't want to die, or have cancer. I feel like i'll be one of those cancer stories were the child has everything going for them in life, smart, knows what they want to do in life, and then there struck by a terminal illness. Thats my biggest fear right now. The knot is kinda making my chest hurt alittle, its not really big, its not a pimple, or anything like that, its under the skin, its a knot I think. I had my mom feel it she said to just watch out for it to see if it gets smaller, or bigger. It doesn't hurt when I press on it. I know Yahoo answers is not the best place to ask a question about medical stuff, but it has fast responses. I'm just really really scared right now.