I've been having sense of fullness in my left leg behind my knee, its obvious I should see a doctor but in the meantime are there any over-the-counter leg pain reliever?

Let me try to help. Although I have no definite diagnosis, you could some Motrin and apply moist heat on it , avoid squatting and strenuous activity involving the knee. Please check it out as soon as you can. Than you.
Knee fullness. One possibility that dr. Hadied, as an orthopedic surgeon, would know very well that can cause a feeling of fullness behind the knee would be a bakers cyst. Ibuprofen and moist heat could help that, as he said. Something that could be hard to tell from a bakers cyst, that you don't want to miss, is a blood clot. Large bakers cysts are sometimes a nuisance. Blood clots are potentially deadly.