What causes lower leg pain on running?

Shin splints. Shin splints is a common cause. Other causes can be tendinitis, muscle strain or fatigue, stress fracture, or compartment syndrome.
Many causes. Common causes of running related leg pain include medial tibial stress syndrome (commonly known as shin splints), stress reaction or stress fracture of bone, muscle-tendon injury (strain, tear, tendonitis, tendonosis). Less common causes include exertional compartment syndrome, nerve or vascular entrapment, fascial hernia, exertional dvt, synostosis, etc. See a sports med specialist.