What Causes Intermittent Breast Pain?

Hi am 24 years female who is working and studying at the same time. I have been suffering from pain in the upper part of the breast. Its not continuous pain but occurs sometimes. I did do not have any lumps and not sexually active. I go for Yoga class everyday. I've an average body. Am kinda shy to go out and ask a doctor so i thought of using this Medium.
Thank You.