Could be a number of things. Are you in flower? what setup are you in hydro or soil? What is ph at? What nutes and at what strength or ppms are you using? Pics?
i am in flower been in 5 weeks and im using 2x 600w hps on 8 plants the jellow has only started from the flower and im using soil canna if uve heard of it its good quality in england
Usually not this early in flower though, is it alot of the leaves or just a few fan leaves? If it is just a few leaves than it could be natural for a 8-10 week flower strain, i still think 5 weeks is to early to be nitrogen deficeint