I have a pain in my lower left leg, what can this be the result of? A circulation problem?

Several things. Some are: phlebitis (inflammation/clot in vein of leg), arterial--vascular--insufficiency, muscle strain, tendon or ligament problem, degenerative process in knee or sac around knee known as bursa, which is inflammed (bursitis), forms a cyst (popliteal cyst), then ruptures, causing pain and swelling down calf and leg. This last cause can exactly mimic phlebitis. Needs doctor's hx, px, dx, rx.