There are several different causes of pain in the leg, but since this has been going on for two days now with no obvious source, I think it is worth going to see your pediatrician
for an evaluation.
Most of the time, this kind of pain ends up being from a minor twist or sprain, from running or playing sports. However, you should mention to your doctor
when you go to see them if there have been any falls or other events that might have resulted in trauma to the leg. If so, this might make your doctor lean more towards getting some x-rays, as might any positive findings on the physical examination.
Also, in children, unexplained leg pain is sometimes a sign of osteomyelitis, which is an infection of the bone caused by bacteria. Make sure to tell your doctor if you have noticed any fever, as that would go along with a potential diagnosis of osteomyelitis. Your pediatrician may want to obtain some basic blood work looking for signs of inflammation or infection to rule out this possibility.
Start by contacting your pediatrician's office today! They should easily be able to help you figure out whether this is something that you need to worry