What Makes My Calf Swell?

My right calf has been swelling off and on for about a year. There is no pain but rather mild discomfort and sometimes numbness. I have been checked twice to rule out blood clots or poor blood flow. The leg tends to swell after sitting for a period of time but can also swell up when I am walking. The swelling used to last for about 20 minutes and then go away but lately it stays swollen for hours/days on end. Any thoughts?