What Causes Knee Pain Along With Difficulty Walking?

I was traveling from Indy to Seattle and when I got off the plane, I noticed my knee was tender and it was hard to walk. I sat for about 5 hours without moving and I had knee high stockings that cut off my circulation a bit. I ve been favoring it now for about a week. When I walk a bit it loosens up and after I sit it gets stiff. Any suggestions?