I developed blood clots which led to pulmonary embolisms twice within 2 years. The doctors never had a definitive answer what caused them and therefore I am on blood thinners for life now. It was one of the scariest times of my life. The 2nd time I had the PEs that traveled from my legs, the doctor told me had I gone for a run and not the ER, I might not be here. Very scary! Every time I get calf pain or shortness of breath I always have it in the back of my mind if it's another blood clot/PE. And it's hard not knowing the "cause".