Bright yellow new growth..

Hi All,
I have yellow growth on new leaves, these have just grown and turned bright yellow overnight. It's a Royal Haze Auto germinated from seed and planted out in to a polytunnel and was doing quite well until the yellowness... up to this point i had not fed them, as i've heard that autos don't need feeding, i flushed her with 2 gallons of water and then fed an N-P-K feed (Phostrogen, i swear by it!) over next couple of days the leaves got a little bit greener in colour, presumably it was taking on Nitrogen? the yellowness has made the new leaves very thin as in photos... Grown in Compost with added John Innes no 2 which i have always used and had some great grows... i believe it is lacking some nutrient but which one? any help or opinion on this would be very well received. plant is getting ready to flower round about now as i have a few others that are already budding up.

 

Thanks for quick reply... Iron... hmmm hadn't thought of that... is it curable?

 

Thankyou for this, fingers crossed

 

Thanks people, i'll check my Ph first and see what we get ...