Hilary,
I can relate. My periods were so irregular it was hard to keep track of when they whould be due. If you have taken several tests that have turned out negative, then I would guess that you are not pregnant, but I can understand your worry and concern until your next period comes. I had a similar pregnancy scare several years ago. I went to the doctor because I wasn't convinced. It turned out that I was not pregnant and eventually my period came. If you are still worried perhaps you should see a doctor to find out what is going on. In the meantime try not to stress too much. Good luck.
Kate