Despite having had the whooping cough vaccine as a child, i still got a mild case of it at age 7. how does this happen?

It's not one shot: Even when you were 7 to be protected you needed a series of shots. Perhaps you did not get them all. Vaccines are pretty good protection but they are not 100 percent. There are vaccine failures. I'm guessing the vaccine you did helped limit the severity of the case you did get.

How diagnosed?: Immunity to even repeated pertussis vaccine can wane. Admittedly, at age 7 most children that have had a full series of Pertussis vaccine (TDaP or similar) are protected from Pertussis. You were young, but was a Pertussis swab obtained from your nose? Any antibody tests done to check your immunity since then? I say this because it is impossible to diagnose pertussis accurately without those.