Well, it appears that you have some type of "hypercoagulable state" -- meaning there is a propensity of your blood to clot abnormally (e.g. more likely to clot).
These clots, presumably, formed in the legs (called deep venous thrombosis, or DVT) and traveled to the lungs, causing the clots in the lungs and trouble breathing (pulmonary embolus).
There are a number of medical conditions which could cause this, and the first step is bloodwork to help and narrow this down.
Protein C deficiency
Protein S deficiency
Factor V Leiden
are all different blood conditions that can cause clots to form. In addition, there are other conditions, such as recent infections, injury/trauma, cancer, pregnancy, birth control pills, etc that could have also contributed to blood clots.
So the first step is to do the bloodwork to figure this out, which your consultant (a hematologist is the specialist in this area) will have to evaluate.
Here is a medical article (in medical language) about the topic:http://www.clevelandclinicmeded.com/medicalpubs/diseasemanagement/hematology-oncology/hypercoagulable-states/
And here is one, more for the regular person:http://my.clevelandclinic.org/heart/disorders/vascular/hypercoagstate.aspx