Does it hurt to flex or extend your toe? Do you remember striking the toe on anything? What type of running activities have you been doing? Just straight ahead? Turns? Stop and start? Jumping?
If you struck your toe on something, you probably want to get an x-ray to rule out ony fracture. If you remember no trauma to the area, then it is more than likely a soft tissue sprain/strain caused potentially by your mechanics, shoes etc.