"Not my cup of tea" -- pretty advanced English idiom. Shows Petra's increasing facility with the language.
I don't think so that it is so advanced, it's the English version of the idiom that we use in The Czech Republic and we also have a similar version with coffee instead of tea ("Not my cup of coffee." = "To není můj šálek kávy.") which is more common.
But I do think so that she's increasing her English, she did understand better than me (but she has the opportunity of hearing the person in next to her).