Yellowing of the leaves is normal midway to the end of flowering. The plant is using up its stored energy. It should be a gradual and not rapid yellowing. Have you checked you PH?

If you believe you have the grow under control then its most likely natural yellowing in the flowering stage.

If you dont think it is then copy and paste the troubleshooting form from the plant problems section and fill it out. It will be much easier for people to diagnose a problem with the form completed.