I presume that this is just one swollen leg and not both?
In which case, as blood flow and clots have been ruled out, this only realistically leaves a condition called Lymphoedema, which is a blockage of the lymphatic channels. Cause is unknown but gland involvement in the groin and in the pelvis must be excluded by a scan.
Difficult to treat and "water tablets are of little or no use in the treatment.
It doesn't get better and the important thing is to massage the leg with E45 cream to maintain the condition of the skin.
Often seen in obese patients.