There is no evidence for decreased mortality in screening women under the age of 40. The purpose of starting screening mammography early in the 40s is to detect early breast cancer. There hasn't been found to be a significant number of breast cancers found in women under the age of 40 to substantiate the need for screening before the age of 40. Many younger women have benign processes that can be evaluated as they come up. That is a very common symptom prior to menstruating.
Many women complain of breast pain prior to menstruating.
This has mostly to do with the associated hormone surges. If the pain is persistent, of course, this should be evaluated by your physician. But most radiographic imaging does not explain breast pain and it is a very common symptom especially if it is cyclical. The best time is seven to 10 days after your period. Of course annual screening mammography should be performed after the age of 40.