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Do you smoke? Have any other medical problems? Diabetes? High blood pressure? Taking any meds?
One possible cause is a deep venous thrombosis which is a clot in a leg vein. This is potentially dangerous because the clot can break off and travel to your lungs causing inability to breath. There are many other possibilities, but you should have this evaluated by your doctor to be safe and since I cannot examine you in person I cannot know how likely this is. I would recommend speaking to your doctor asap or getting to the emergency room where an ultrasound can be done on your leg to see if blood is flowing properly through the vein. If so, many other less concerning possible causes can be examined. here is a link with more information: http://www.uptodate.com/patients/content/topic.do?topicKey=~zEdkCtNwwHHJMg
I hope this is helpful and please let me know if you have more questions.
I would recommend having this evaluated asap as I cannot know the degree of seriousness without seeing you in person. Therefore erring on the side of caution, I'd recommend having this checked out as soon as possible. But you are welcome to speak with your GP and wait until Monday if you feel comfortable waiting.
Best of luck and hope you feel better.