Kayleigh, as PJ says, you do not need to be worrying that this lump might be cancer, but it could be some sort of infection. If you have read the links that PJ suggested, you know that breasts experience "growing pains" during puberty, so a sharp pain and some lumpiness can be perfectly normal. However, if your breasts have finished growing, this pain may need a doctor's attention. Let an adult know about this pain and help you decide if you need to see a doctor.
Hi Kayleigh - No doubt you're scared about this. Please read our message to teens, and our guide to breast development. I'm sure after reading those, you'll feel a lot better. Bottom line: chances of you having breast cancer at your age are WAY less than 1 in 1 million. Take care- PJH