I'm always thinking I have a terminal illness and that I'm going to get Cancer

I can assure you that you DO NOT have cancer 

It sounds like you may have nervous symptoms. If your bloodwork is normal and the doctors say there is nothing wrong then you are overloading your mind with worry, which we all tend to do at times. Diarrhea can be caused by diet, worry, etc. I have back pain but I know it is due to disc problems that I have had for years. Fear of that which we can't control can be consuming. Sometimes we think we look for symptoms because we don't think we are good enough to not have something wrong, in other words, shame perhaps. I understand fear of symptoms and blood work, going to the doctor, etc. It is human to worry about things, some are better at it than others, and some have real reasons to worry. Worry can be a substitute for something else. Do you have family members who have had cancer? Still, wasting time worrying is a substitute for not doing things worthwhile that help us to forget ourselves. Are you alone a lot? We can benefit from Dr. Net, but it also helps to create OCD when people have to check everything symptom they have, I am just as prone to do so at times. If you are as consumed as you say, then your symptoms are emotional, worry is exacerbating the problem. I don't know how old you are, but I started with back problems when I was 16 and have had pain for years, but tried to work out, keep weight down, etc. Girls may have back pains for other reasons, but your doctor should clear that up. Youtube and the Net are good resources, but they can become addicting. Imagine not having the Web. You would have to go to doctors or look in medical books, but information was limited then. Now we have everything wrong with us, so we think. I hope you are able to come to terms with what you have and what you don't have. 

Rebecca I am exactly the same my anxiety returned worse than ever last year after I hurt my knee and of course my mind was imagining every illness I googled every illness and cancer always came to mind.I realised it was nothing serious so with difficulty I started to do my breathing  and with time I started to calm.It did not help when a lot of people I knew were dying.But just hang in and you will calm .Please stop googling.

Gooe is your worst nightmate for health anxiety believe me been there done that and got the t shirt!!