Why did i get thigh/calf swelling out of nowhere?

See your doctor soon: One of the worst things we worry about with one-sided calf or leg swelling out of the blue is a blood clot. If you haven't had any trauma recently or think you've pulled a muscle, you should see your doctor. Other things include a ruptured baker's cyst (a fluid filled sac behind the knee), or a ruptured head of one of the calf muscles. Best of luck and take care.