Need help!!

So I am 8 days late on my period! This is the third month I've been late. The first month I was 6 days late then bleed for 2 days and the very light spotting for 4 days. The second month I was 3 days late and bleed for 5 days. Mind you before all of this my period ALWAYS and I mean it has ALWAYS came on the first for 5 days with normal bleeding. I've taken 3 pregnancy test and they've came out negative. My nipples are very sore to the touch and I've had some crazy cramps! And I'm extremely horny especially when my fiance comes home from work. Has this haapen to you were the pregnancy test just didn't pick up that your in fact pregnant???