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.