I have lumps in my breasts, does that mean I have breast cancer?

Whoa, sorry for that title. It's the best I could come up with, for some reason. Anyway, I am currently seventeen years of age, and I'm a female. I can't seem to find any information anywhere that can answer my question, so I hope someone here can. What kind/size of lumps does breast cancer include? I always hear, "Oh, I felt a lump and it was cancer", but no one ever specifies what it feels like, so I don't know what I should be feeling for. I first noticed these lumps when I was thirteen. (I don't remember anything specific, I just remember the age.) My right breast has several, (almost the entire mass of it, really), lumps in it. They're not like tiny lumps, they're of a decent size. My left breast has two large ones that take up nearly the entire breast. If I apply enough pressure to feel the lumps clearly, it causes enough pain for me to be uncomfortable. Any help or information you could share would be wonderful. ^-^