My girlfriend is talking about marriage

my girl and i have been together for 3 years and we love each other 2 death. so she always brings up the subject of getting married. 2 years ago i thought."WOW, THAT WOULD BE GREAT!" but now, its a turnoff for some reason. i told her we should at least wait till after college.till we're like 23-25, when we could support ourselves and a family. y does the marriage topic turn me off? i dont want 2 make her sad by telling her "NO i dont want 2 get married right now"...is this normal? im just thinking about having fun with her bc i love her and i never put ANY thought into marriage bc it scares me now..im only 18..plz help