What do you think of my theory of how I might have generated blood clots?

I’m a 62 year old man who spent 9 days in the hospital (7–8-13 though 7–16-13) for heart surgery. After I was released from the hospital in Indianapolis until I flew back to my home in Seattle (5 hour flight) on 7–22-13 I felt a bit weak but otherwise good. For 5 days in Indianapolis I did a lot of easy walking (as directed by my surgeon) per day. It felt so good to walk so I walked 3 to 4 hours a day very easy while wearing a light backpack (10 to 12 pounds). When I got back to Seattle I felt very good for the first 3 days (7–22-13 to 7–24-13). I started to do my calisthenics, swam twice for 20 minutes, did a very easy 20 mile bike ride, did a lot of yard work and went for a 7 mile hilly walk with my wife all with no problem. My life seemed to be getting back to normal at last. However in the evening of 7–24-13 I experienced very extreme pain in my upper right and left shoulder muscles (where the backpack straps were). Later that evening the pain radiated down to the areas of my heart and lungs. My heart was pounding and it suddenly became difficult and was painful to take a deep breath.

I went the the Emergency Department of a local hospital where they found blood clots in my lungs (Pulmonary Embolism) and was treated in the hospital for the next 5 days and released.

The doctors in the hospital were sure that I initially formed the blood clots in my legs. However I never had any pain, numbness or cramps in my legs. However when I offered to the doctor my thought that the blood clots most likely originated in my shoulders (because of the constant and long term pressure the backpack straps had on veins my shoulders could have facilitated clot formation), she just laughed at me. She explained that blood clots typically form in the legs. I said that’s true but I have had no leg pain and a lot of pain in my shoulders. She just emphasized that the legs is where blood clots form and ordered a doppler ultra sound of my legs which found no clots.

What do you think of my untypical and out of the box theory?

FYI: The very intense shoulder pain is still with me after the second day out of the hospital. Also I’ll be seeing my Family Practice doctor tomorrow.