the only thing i can think of is, an ingrown hair?
at you age you body is going though lots of changesyou homones ae changing you body is gowing fom a child to a young woman,the lump could be a gland,go to get it checked if you ae concened,speak to you mum about it so she can help you.
Do you drink a lot of caffeine type drinks? Some women develop cysts that are from drinking tea, soda, or anything w/caffeine. Some people also get what is called a lymphoma (fatty tumor) these are usually benign. Can you move it or is it fixed to one area? To be positve and if it bothers you, you should make an appt. and have it checked out?