Kristina, what you are feeling is almost certainly a breast bud.  It's the sign that you are entering puberty and that your breasts are starting to grow.  You can read more about what to expect as you go through puberty in our Teen Guide to Normal Breast Development.  Since this ball is on only one side, it is also possible that you have an infection, so let your mom or another adult know about the problem.  She can make the decision about whether you need to see a doctor.  One thing you do not need to worry about is whether this ball is cancer.  It's really good that you are paying attention to your body and doing some research to find out what might be happening.  Keep in mind that what you read on this site applies mainly to grown women.  Talking to your mom or a doctor should ease your concerns.