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Breast does not contain muscles.
It contains only fibrous tissue, glands, ducts and lobules.
Breast swelling during nursing can stretch the skin over the breast. Then when your breasts shrink again, you have this loose skin that appears to sag more than it did before.Breast pain
can be cyclic, coming with menstrual periods, or it can be more persistent.
so most probably it could be a cyclical breast pain.
Just sagging breast does not cause pain.
You can prevent sagging by
1. Wear sports bra during sports. It prevents stretch of ligaments in breast.
2. For most of women, bra wearing can actually increase sagging.
Hope this helps.