Welcome to HealthcareMagic forum.
could be due to hormonal
reasons most of the times , it is common during the time of expecting cycles,but there can be other causes too, in inflammation of breast, trauma,infections like mastitis
, or sometimes drug effects may cause pain. You must also check for any mass, discharge or nipple changes by doing a self examination of both the breasts.
Take a pain killer for now , check your breasts for above, if you have fever and redness
around, or discharge, do see your doctor soon.