PLEASE EASE MY MIND!

Im currently 3 days late on af I took a pregnancy test and it came back negative, I've never been late before and I'm not in the slightest bit stressed, was it just too early? My nipples hurt so bad, Ive had a headache for 5 days straight, and when I talked to my nurse she said it could be to early to tell but how? Im literally 3 days late, it makes no sense! Please help!!! Has this happened to anybody else, and then later they have gotten a BFP? Please ease my mind..