The leaves on my plants are curling up and starting to go wrinkley plz plz help?

i have 3 cannabis plants crowing and they are two weeks old they each have 4 leaves on them so far but over the past 3 days the leaves have started to become a lighter colour and curl up (towards the light) and wrinkle i have them growing in miracle grow soil..............does anyone know what could be causing this plz plz help