Looks like your plant has Nitrogen defficiencey. The yellowing is on the lower leaves. What the plant is doing is moving the nitrogen up from the lower leaves and bringing it to the top where the new growth is. What type of fertilizer are you using? You need to up the N in the soil. Piss a little in a cup and dilute it with 2 parts water and hit your plant up with it for a quick shot of N, then start using some high nitrogen fertilizer every week