Help Im having upper calf lower thigh frontal knee tightness and a little swelling. What is this?

I have been having this discomfort on and off for 1 week or so. It started in my lower thigh with some pain then now it's in my calf and front of my knee and it feels tight when I sit down or lay down. What is this? I was in the ER for a seperate problem (breathing) the doc asked if I was having leg pain I said yes for a few days in my calf and lower thigh area so he drew blood and said there isn't any clot, so what could this be?