It's called puberty. Welcome to womanhood.
Don't worry, they will likely continue to be one of your most "impressive" bodily features until you either have a child or become another notch on gravity's bedpost. Supportive undergarments such as brassieres or duct tape can help alleviate any discomfort you might experience from them as they continue to grow, while conferring a firmer, more youthful appearance to them as you age. As well, various designs are available which will enable you to continue exercising without them flopping and contorting like angry ferrets in a pillowcase.
As you continue to mature and realize that teenage boys are not slugs, snails and puppy dog's tails as you once believed back when you played hopscotch and sold lemonade, but instead are manipulative, acne-ridden erections reeking of awkward, mid-pubescent BO smothered in Axe cologne, you will soon find yourself fascinated with them and will likely find it useful to invest in more aesthetically appealing apparel. Various designs are available which, while supportive, additionally serve to enhance shape and appearance while accentuating cleavage. Note, however, that every time you wear a padded bra, God kills a puppy, uses it to club a baby seal to death, uses both carcasses to smother an orphan and then feeds all three to the homeless in a chili con carne dish before sending Radiohead over to tell them what a bunch of cry babies they are. Don't be a deceptive cunt; save the Kleenexes for drying your tears after that boy who promised he loved you doesn't text you the day after. In your bra, the only good they will do is cushion your fall after he kicks you to the curb for false advertising and tells everyone at your high school what a slut you are. Oh, and the front clasping types are always a welcome treat, IMO. I dunno, the reveal is just very cinematic, like the curtains opening at the premiere, though instead of paying for a ticket the price of admission is some booze, acting like I give a shit about her life, pretending there's a sense of humor behind those tits and a whole lot of lying about how much we have in common.
If you have any questions, I'm sure your mother, older sister or even pediatrician might be able to offer better insight than a bunch of dudes on a bodybuilding site. Note that, as you continue your journey into womanhood, your body will experience additional changes as well. Bloated, bloody, irrationally emotional, chocolate-eating changes which are not only beyond my area of expertise, but just totally disgusting. But, don't worry, boys will feign sympathy because of your budding, "impressive" bodily features. Again, see your mother or physician for clarification.
When you turn 18, shoot me a PM for additional instruction. Good luck, OP!