I have a good friend that does the same thing. For her it seems to be a combination of things, weight, hormones, time of the month, and birth control.
Iv watched her go from solid C (on very small petite frame) down to an almost A then back upto a barely D. when down to her smallest, she was probably 95lbs, the solid C at about 110lbs and then the D at about 105. changes in her bust dont always go with her wieght, the other things factor in.
It is just her genetic makeup I guess. So Its not unheard of.