You can put ice over the area to reduce swelling(if present) and pain. You might not have jumped or placed your foot in a wrong way but still there are chances that you have injured yourself. It could be ligaments, meniscus or any other structure in the knee joint.
Next Steps
It is advisable to consult any Orthopedician or Physiotherapist to know the exact issue. They would examine you and perform a couple of clinical tests to diagnose the injury.