My red blood cells are elevated.... What could cause this?

Last year I had an episode where I was hospitalized due to a possible blood clot. My symptoms were shortness of breath, jaw pain, and extreme fatigue. I was given blood thinners and sent home. Now nearly a year later, I am having similar symptoms, ie shortness of breath and fatigue and bruising that takes forever to heal. My red blood cells are elevated and Im not sure what that means.. Have been told normal range is between 150,000 to 400,00 and mine is in the 700,00... He is going to refer me to a hematologist.... What could this mean and is it serious?