How do i tell my girlfriend that her birth control is killing our relationship?

Okay my girlfriend has started taking the birth control pill that like only every 3 months she has a period, and when she’s on that pill she tells me she’s not happy and she’s rarely sexual towards me, but when shes’ on that one special week of the fake pills she’s very affectionate and sexual towards me. So my thing is how can i prove it’s her brith control and when i do how in the world do i go about telling her that because i’m sure thats a very touchy subject to go about with.