Vanessa,

Under the breast cancer symptom section on this site, there is good guide to how to do a breast self-exam.  The basic idea is that you are looking for any kind of changes through a systematic visual and manual check of all parts of your breast. Because your pain is on just one side, you can compare how the right breast feels and looks to the left side.  There are many possible causes for pain including normal hormonal changes.  If your pain lasts longer than one menstrual period, or if you develop redness, swelling, or dimpling (like the dimples in an orange peel), you need to contact your doctor.  You might have an infection or a type of cancer called inflammatory breast cancer.  Of course, if you develop a fever or your pain becomes acute, don't wait.  Tell your doctor right away.