PMS is one possibility. So is pregnancy. Have you always had this pain, or is this a recent change?
Here are the top ten possible causes of breast pain:
* A change in body lotions applied to the chest: maybe you are allergic to some ingredient
* Brassiere: inspect for defects; try wearing a different bra
* Exercise, especially chest exercises. Maybe it isn't your breast(s) at all.
* Infections: ingrown hair, acne, or internal infection
* Medications, especially the birth control pill: excess estrogen often makes breasts sore
* Breast feeding or breast play. If this is the cause, you'd probably suspect it long before typing the question.
* Trauma: were you struck by something (baseball, door, etc.)?
* Hormone levels. Unfortunately pre-PMS breast pain is considered normal. Each woman has a different strategy for dealing with it from staying in bed to extreme exercise, pain relief medication to herbal teas.
* Rib pain: unlikely due to your age.