its normal for many strains to begin to drop fan leaves during flowering, the plant is taking all the leftover food from the leaves and sending it to be turned into resin, the leaves will yellow and drop off, it is best to allow this to happen at its own rate so the plant can use all of the available food. if you see some sort of infestation or if the leaf is blocking light from a nice bud site you can remove it before it turns dries out and falls off.
i have grown nirvana's white widow and she looked like a late autumn day during flower. expect more leaves to yellow and drop as flushing continues. on a side note, if u use all organic ferts your chances of nute lockout are highly reduced, also i only flush during the last two weeks of flower.