Compartment Syndrome...

Muscles work in groups which are known as compartments. Connective tissue in these compartments isn't very stretchy so during exercise when fluid goes to the muscles and they swell there isn't enough room for them in the compartment so the pressure builds up and creates pain. 

There are a couple of causes;

When growing the compartment didn't grow big enough

A growth spurt as a child where the leg compartments didn't catch up with the rest of the body