What to wear under swim suit?

okay so this is akward but im 13 and my breasts have developed really fast since last summer. i have a birthday party coming up at a pool but im not sure if i should wear a bra under my swim suit. if i get the swimsuits with the built in bra, the bra works but the rest of the suit is all floppy and big. and if i wear it with out a bra you can see my nipples . plz help me i really dont want to talk to my mom unless its really neccesary.