I [M] think about my [F] girlfriends past

We are a younger couple in our mid 20s but for some reason I’ve been thinking a lot of her past sex life and it turns me on.. I enjoy watching herself use a vibrator and make herself cum, at times while having sex I think about her being bent over or her giving oral to someone else and it’s something I fantasize about.. I think it is because I enjoy watching her please herself and turns me on tremendously many times while having sex I have restrained myself from asking her about past sex stories
Is This normal??