What Causes Pain In Right Leg Near Ankle While Running?

Today when i was running, I felt a pain on the right side of my right leg a little close to my ankle. When I walk, it feels okay. But when I run it really hurts. So basically, it only hurts when I put a lot of pressure on my right leg. The only thing confusing me is that there's no mark or bruise where it hurts. What's wrong with me? Should I go to the E.R?