Hi, I'm On Cerazette, And Have Been Now For About

Hi,
I'm on cerazette, and have been now for about 18 months. I have always had regular periods around the same time every month whilst on them. Recently I have been getting very bad cramping, sensitive breasts, nausea, vomiting occasionally, head aches, heartburn, dizzy spells and cravings for the most obscure foods (even foods I do not like such as pizza). Never before has this happened whilst being on cerazette. I have taken 3 home pregnancy tests and I have also have 1 blood pregnancy test done, all have come back negative. I have spoken to a family planning nurse and was told that there was no chance of me falling pregnant on this pill if I have taken them correctly (which I have) and haven't missed any (which I haven't). Is there any chance that even with the 4 negative results I could still be pregnant?
Thank you