Hi Anna - At your age, your breasts are changing very quickly; it's inevitable they'll grow at different rates, with one becoming bigger than the other, and/or developing a funny-feeling lump, while the other doesn't. This is perfectly natural. Please read our message to teens, and our guide to breast development; I think both of those will help you to stop worrying. Also, you might mention this to your mom (embarrassing as it might be!), or another adult; adults like to know this kind of thing, and worry shared is worry diminished, right? Finally, if the lump doesn't disappear within a couple of months; or if it seems to get larger quickly, you might want to see a doctor to find out if something out of the ordinary is going on, OK? Good luck - PJH