Swimming costume- when is it too tight? (Ladies)?

The last time I went swimming I was 9 years old. I'm now 20 and want to start swimming again. The costume I bought is a size 6, it fits perfectly everywhere but around the bust area it feels slightly tight. I don't know if this is normal or if I should return it. My bra size is 30B/32A so I'm surprised the bust area feels so tight because my breasts are in proportion/less than in proportion to my body.