I have lost 8 kg been doing boot camp my breasts are starting to become saggy is this normal and what can i do to prevent this from happening please?

Good luck: normal.... "ineluctable" is a better term. Breasts are fat and glandular tissue with some small amount of supporting structures. They dynamically change size. The fat can dramatically go up and down but collagen/protein/support structures don't. Deflated balloon. Sorry. Worse is there is no guarantee that putting back the weight puts it there and not just around the belly.