Should I buy a bigger cup size?

Okay so I usually wear a 32C.
I have a bra from Calvin Klein and the C is way too tight-my boobs like fall out. Lol. But it’s not tight around the band… And I have a Frederick’s bra in which a 32C I barely fill in, I think because it’s probably a full coverage though. I want to buy another Calvin Klein bra but I don’t know if I should get a 32C or 32D? I have been measured and yeah my size was 32C, but I heard that Calvin Klein runs big? o.O
Thanks.

The best thing when buying a bra is to try it on. I know depending on the bra I can fit from 38C to a 40D.

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