Why do I sometimes have pain in my lower leg but not any pain above the knee?

Localized lower leg. Pain could be muscular or vascular. From shin splints to blood clots. Persistent symptoms should be evaluated by your physician please.
Leg vs back. Leg pain could be referred from your back or it could be locally from the knee/ leg. Local problems include bursitis, strains, stress fractures, "shin splints", and tendinitis. Occasionally people can have nerve entrapment syndromes in the thigh/knee/leg leading to pain in leg. Varicose veins can be painful and rarely a blood clot can repoduce leg pain. See a physician for proper evaluation.