It sounds like you have had a doppler study to rule out a deep venous thrombosis, which is good.
I suspect they have checked the lymph nodes to see if there was any lymph node enlargement which could be causing problems with blood flow "from" the leg. If you are not sure if they checked this, ask your doctor if it says anything about that on the report.
Anything which blocks blood flow from the leg (i.e. from the leg back to the heart), can potentially cause swelling of the leg.
If you are overweight, this may be part of the problem.
Other possible causes for this leg and calf swelling may be superficial phlebitis, varicose veins and lymphedema.
Another cause can be some sort of inflammatory response to arthritis in your knee. An MRI would be helpful to rule this out.
For people with mild lymphedema, compression bandages can reduce the swelling.
People who are more severely affected may wear pneumatic stockings every day for an hour or two to reduce the swelling. This would have to be prescribed by a doctor. Once the swelling has been reduced, the person must wear elastic stockings up over the knee every day from the moment of rising until bedtime. This controls the swelling to some degree. Ask your doctor about these possibilities.
I hope this helps you!
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