PLEASE HELP with my leg?!?

I have had a lump on my knee for a few months and the doctors have never seen anything like it before. Today my knee suddenly swelled up and the muscle in my shin underneath my knee has swelled up but it is rock solid. It is quite painful. I haven't done any sports that would cause this recently. I have had people say to me it is cancer but as a joke so I did some research and bone cancer is very common in teens. Does anyone know what it is and am i overreacting? I think i am! My mum said she would take me to the doctor in a week if it is still there. PLEASE HELP?!