I continue to get blood clots even after being on Warfarin and Lovenox shots. What should I do?

I seem to be prone to blood clots. After getting my first DVT (I was 19) I was put on warfarin. I was in therapeutic range when a year later I got another clot. My hematologist decided to put me on a lovenox / warfarin mix and I clotted again. They decided to give me more than double the dose of lovenox and I clotted again. Now being 21 and having four blood clots (two of which were DVT's [one in each leg]) my doctors are extremely concerned about my life. They have done several tests on me trying to find out what disease or illness I may have and they can't find anything including cancer. I am looking for suggestions on what I should do. I even went down to Duke University's Cancer Center to get checked out and they didn't help at all either. Does anyone have any suggestions or relating stories on this type of situation?