Definitely get it looked at. I get swelling in one of my feet/ankles, but its from DVT blood clots cause by a genetic abnormality. My PCP recognized it immediately when he walked into the exam room and saw my feet dangling over the edge of the exam table. Not saying you've got clots, but as others mentioned it could be edema that's causing fluid build-up or a muscle injury that's not healing properly. Get it checked out.