You could either have a non threatening case of Edema:
Swollen feet and legs, referred to medically as edema, occur when fluid is retained in the spaces between body cells. Edema typically affects the feet, ankles and lower legs, but can also impact any area of the body, causing systemic symptoms.
Techniques to help decrease swelling:Increase muscle activity with walking. Avoid standing in place for long period of time. Avoid sitting with the feet dependent. Limit salt intake. Drink plenty of water. You say it's been 6 hrs. Have to been sitting during this time? It's won't help the swelling go down.
Life threatening would be what they call Thrombophlebitis
My legs have been swelling for a good month now. More so during the evening. When I finally went to the Doctors the 1st thing she had me do was an ultra sound on my leg to rule out Thrombophlebitis. Thank my lucky starts there was no clot. Due to my job of sitting a good day of my way away it is a bad case of water edema. I have started on some meds to reduce this. But for me lying down and keeping my legs elevated has worked too. So if you can to be on the safe side, I would get checked by your doctor or at least call the answering service and let your doctor know what is happening. They may set you up with an ultra sound or call in a water pill for you. Good luck, hope this helps.