I love you during sex?

Well last week I was having sex with my girlfriend and when we finished I layed down next to her and she look at me and told me "I LOVE U" so I was kinda quiet for a bit then kissed her softly and kept silent......but I dont wanna tell her I dont lover her but I just think its too early!! and more so after sex! ok maybe I just wanna have fun with her and hang out but im not ready for a serious relationship with her. just live life and have fun ............. so what can i tell her ? or what can i do ? to end this in a formal yet discreet way ? shes really in love with me now and I dont want to make her feel bad or dissapointed !!!