I am a cardiac rehab nurse and one of the things we tell our patients is that you usually don't think twice about your heart, but once you have a heart attack or anything else related to the heart, every pain you have between the top of your head and your belly button, you are afraid it is your heart. That's normal.
I think the same thing happens once you have a diagnosis of cancer, but it happens on an even greater scale. While your goal is to go on with life once your treatment is completed, there are always side effects that hang around, there is always the fear of what that next PET scan or blood test will show. That is normal as well- you have just been through a major life changing event.
Just remember: Life is not about surviving the storm but learning how to dance in the rain.