Yellow Spot

Hey, im about 3 weeks into growing some very nice plants and one of the plants has a really small yellow blot growing on it towards the edge of one of the leaves.

I would take picture but i dont currently have my digital camera. Just curosus as to wether it is a big problem i need to worry about and could it spread to other plants?

~Thanks in advanced ~

I've attached a few pictures to show you how the yellow spot looks on the leaf. Can you tell weather its been under watered or over watered?