strong positive, then a bunch of negatives, still 4 days late ?!?!

Hi! I took a pregnancy test after being 1 day late for my period. It was positive. It turned positive within 60 seconds and was a VERY strong line. I took this test in the afternoon. A few hours later, I took another and it was negative. I assumed it was because I flooded the test with WAY too much urine. I took another test with first morning urine the next day and it was negative. Not even a hint of a line. I am now 4 days late and still getting negatives. I only got that one positive test and then hours later they were negative. I "feel" pregnant and have had symptoms. For instance, I had 3 days of implantation cramping. the timing is right and was followed by 2 days straight of a bad headache. I have had some milk nausea, too. I feel unusual in the general area where there would be a new baby forming. This would be my 3rd baby, so I have had this feeling before. I just don't understand why I am getting so many negatives when so many things point to me being pregnant. It doesn't seem like it would be a chemical. I had one of those before. With that one, I tested way before my period was to arrive and it was only very faintly positive. This positive test was BRIGHT postive and was taken after my period was already late. What gives?!?!