Do I need to worry about cancer?

I am 21 and over the past 2 years I have developed at least 4 blood clots. Two being massive DVT's. I have had numerous blood tests and cat scans of my chest lungs and heart to also check for pulmonary embolisms and one scan of my head to check for clots and activity in that region as well. My doctors say that if I had cancer it would have shown up on the chest CT's and the blood tests but the more research I do I keep on finding that the cancer could possibly not show......? If this is so, or if the there is cancer or bone marrow tests I should get, what are they? I am going to a new blood and cancer doctor tomorrow and just want to be prepared with questions on what tests I should be getting, etc. Thanks again!