When I was only a few years older than you I too felt a lump which was painful and just like you I feared it might be cancer. I made an appointment with my doctor and I was very relieved to find out that it was what is known as a benign cyst. I had a small operation to have it removed and I never had any more trouble on that side of my breast so please do make an appointment with your doctor as soon as possible to put your mind at rest.
Just to echo what Sue has said, make an appointment with your GP to get it checked out for peace of mind. It is quite likely that it is hormonal and linked to your monthly cycle, it is very unusual for it to be anything more at your age.
Hi Lucka, my niece used to get lots of cysts when she was your age and I remember feeling a cluster of them by her arm pit , they were pretty big and would become painful during her period, now she's in her 20's and has had a baby they have dispersed , see your GP to help put your mind at rest but the chances of it being anything to worry about at your age are very slim Xx
There maybe lots of different reasons for the lump. It could be a cyst, or an infection. Please make an appointment with your gp as soon as possible, to put your mind at rest. It is very unlikely that this is anything sinister, but the best thing is to get it checked.