Started taking evening primrose oil and my boobs are really full-- why is that?

Shouldn't happen: It has been used to help with breast pain but should not cause breast enlargement or fullness. Perhaps there is something else in the supplement that you are taking. I would look for another cause for breast fullness. Evening primrose oil is rich in omega-6 fatty acids, essential fatty acids that are lacking in some people's diets. Always ask your doctor before taking supplements.