It's been 2 days since nipples started hurting and they seems bigger, i am not pregnant for sure. im on my regular period every month, anyone can explain why?

Hormone changes?: Premenstrual syndrome (pms) is common - it is estimated 75% women have some pms symptoms. These include mood swings, tender breasts/nipples, food cravings, bloating, fatigue, difficulty sleeping or concentrating, irritability and depression. Pms is caused by changes in hormone levels (like estrogen) and brain chemicals (like seratonin). Stress and poor eating habits can make pms worse.