What do you think is causing my irrational fear of cancer? everytime i'm even a little sick, i assume it's cancer every single time.

Learn about yourself: The fact that you recognize this pattern of thought as irrational sets you apart from the large majority of humankind who will never achieve much insight. You have a fantastic opportunity to explore this belief and how it arose in the setting of focused psychotherapy. This is likely to be key which, when understood, will give you the control you deserve over other aspects of yourself as well.