What comes after a 36D ?

okay so I'm a 36D but I am noticing that my cup size is too small .. I had tried going up the band size to 38 but that is too big on me. So I do not know which I should do, I do not feel like I am a double D but 36 D is too tight and my cup doesn't seem to fully fill as it tends to stick out the sides. I feelt like my band size is just right but I am not an expert. Can anyone help me?