Hey Weedy....first of all, nice plants....:thumbsup:

You should relax, there is nothing wrong whit that plant....it's getting old, but it isn't ready for harvest yet, believe me...yellowing is a normal thing when so far in flowering, bloom ferts. contain less N than grow ferts., and this is causing the yellowing....You don't want much N in the plants when harvesting, it makes the smoke harsh and it takes more time to cure....

Other thing is that the plant is probably rootbound too...