She told me she is single, should I believe her?

I was talking to a girl last night on the phone and the topic came up and ended up telling me that she doesn't have a boyfriend. Today some guy wrote on her fb wall " Hey baby just stopping by to say I loooove you." She replied to him saying "Awww I love you too babe. Can't wait to see you." She told me she is single and then I saw this. Should I be worried? Am I just overeacting? She did tell me she didn't have a boyfriend. Do male and female friends talk to each other that way sometimes? I don't get this.