I have had sore, itchy breasts for three months. It's getting worse. Nipples look larger & darker. No rashes. No lumps. Neg hpt. What can cause this?

Breast evaluation. At age 21 it is highly unlikely that this is a serious condition. You can schedule a visit to have your nipples checked. Weight changes, diet, caffeine intake, ill fitting bras are all causes of nipple itching. There are many nipple creams available at the pharmacy for itching.