Breast development begins about the age of 9-10. Breast budding is usually painful and occurs simultaneously on both sides.
Wearing appropriate underwear and bras will make you feel more comfortable while exercising. You should make sure that the bra is made from natural fibers (cotton, silk, etc.), the size of the bra is not too small to irritate the skin and not too large that makes the breasts move inside it easily. Remember to take off your bra while you sleep for better blood flow and easier sleep. The shape and size of the breasts change over different life stages. Usually the size of the two breasts is slightly different and not exactly the same.
If you have discharges from your breasts, you should seek medical counseling, examination and health services. At the end of puberty, breast development is complete.