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you could have swelling of a baker's cyst behind your knee which can press on popliteal nerves behind your knees and cause pain in your lower leg.
sometimes these baker's cyst can pop and leak seroma fluid into your joint space and cause pain.
you could have inflammation in your knee which can cause pain and pressure on the peroneal nerve which runs on the outside of the knee.
you should see a doctor and get a knee ultrasound to check for baker's cyst and fluid.
you might benefit from a knee MRI scan.
at your age, a blood clot or DVT would be unlikely and would tend to cause both pain and swelling in your lower leg.
you can try to ice your knee to help decrease pain and inflammation
do you have a doctor you can see who can order some tests and examine your knee?