I Have Had A Dull Aching Pain In Right Leg

I have had a dull aching pain in right leg for 3 days. The pain seems to radiAte from behind the knee downward. It is uncomfortable to cross this leg at the knee over the left leg. I am type 2 diabetic which is being controlled very well thru medication. This leg feels tight at times but I have not noticed any swelling. What might be contributing to the discomfort of this leg?