There is a lump on my neck beneath my skin?! Help please?

Its on the right size about the size of a nickle i think. It is oval like. I do get sore throats alot but im pretty sure i dont have one know. Im almost 13. It doesnt hurt. Im not sure if it is getting bigger or not. Ive noticed it like 4 days ago. It may or may not be growing. I dont really want to go to the doctors because i dont want to be weighed. I hate my weight and im think im developing bulimia and anorexia. Im always being compared to my older sister ( i weight the same) and im littler. PLEASE HELP =[ Im worried if i have cancer or something.