Right I'm 16 but I'm sort of small for my age but my breasts ae growing at a regular pace, but don't know if I need a bigger cup size because you can see them popping out of my cup size I have right now which is an A. Any advice?

You have not mentioned the SIZE i.e 30-A, 32-A, 34-A, 36-A or be it in cms.and so on.? try using B cup with the size that suits your body, or shift yourself to a higher size that you currently wear.

Go into a store that sells bra's and they'll measure you for free, simple as