Boobs

I'm 16 and I'm a lifeguard so I am required to wear a one peice to work and I work basically every day so usually I have no support for my boobs at all( 38DD) I am usually sitting in a chair all day or standing so my posture isn't the greatest but I have been having pretty bad back pain lately and I'm jsut wonder if it's normal to have it Cus I have no support and what I can do to make it feel better...