Leaves suddenly turned yellow and spots along the central vein appeared, can anyone help?

[igm.ig]

Hey,
I checked my plant today and noticed that some of the leaves turned dark yellow and others dtarted to have small spots near the veins. Both are in the pictures below. I haven't changed any settings in my grow box and I'm not sure what's happening (a newbie here). Can anyone help or does anyone know why it's happening?
Thanks a lot guys!

 

[BurnzYzBudZz]

What are you feeding? They look HANGRY.

 

[growsince79]

What are you feeding? They look HANGRY.

+1

 

[LittleDabbie]

+1

+2