What Causes Pain In Nipples After A Menstrual Cycle?

i'm 32 and i have constant very painful nipples. usually beginning about a week after my cycle and lasting until half way through my period. lately my nipples have been chapped and cracking. i've also been experiencing alot of problems with my stomache that i am on antibiotics for. i've also gained about 20lbs in 5 months. i know i'm not pregnant. what could this mean?