I Have Had A High Ddimer For At Least 10

I have had a high ddimer for at least 10 months going from .78 to .73 then I think to somewhere in the .60s now up to .83. I have had 2 cts in that time and one leg ultrasound (never had pain in legs, but have had occ. shortness of breath and some pressure. They decided that it was gastro that caused shortness. Had ekg, recently had treadmill stress test, ultrasound on heart and everything was fine. My pulmonologist says I have some very mild asthma. What could cause this and do I keep having cts everytime my chest hurts. Have had increasing heart rate lately when I take walk.