What could be the cause of my breasts sagging? 17 yo. size c/d cup, gained 15 pounds recently. could this be why? don't wear a bra all the time.

Gravity pulls down: The support structure inside the breasts influences how they hang. These traits are usually inherited, just like other bodily characteristics. On Earth, we have gravity, so some breasts will sag, if they were destined to sag. Without gravity, like on the International Space Station, breasts shouldn't sag, but that's a question best directed to a woman astronaut.