Im not sure but it could be deep vein thrombosis (DVT) which means you have to go to the doctor soon even if it isnt you should go see the doctor anyway to get it checked out.
Hope this helps
Emily
Do u have sensation? Do you have pulses in it. My fist thought would be circulation issues.....any who if you have any type of cardiovascular disease, it wouldn't hurt to get it checked out. Even if you don't have cardio disease get it looked at by an urgent care, clinic, etc. It may not be serious but it's not normal either.
It is possible you have a pinched nerve or disc problem in your lower back either cutting off the circulation to the leg or causing you the cold sensation. Try changing your position every 10 mins if you can. If your sitting, get up and walk for a couple of minutes to see if this helps and /or try doing some stretching exercises with your back. Notify your doctor if the sensation continues. I hope this helps.