Outdoor Sativa dropping leaves quickly

It's my first outdoor grow and I have noticed that my Sativa is starting to drop it's rapidly yellowing leaves after 4.5 months in a 16 gal pot. Buds look very nice, but at what point when all of the leaves fall off will I have to consider starting the flush/harvest procedure? I have been feeding her steadily throughout and currently feed it guano low in N high in P and K every other watering. At what point should I back off of the nutes? I fear that an ill-advised fan leaf trimming around the three month point could have been a mistake.

Don't want to fuck it up now...
Any help is appreciated.

post a pic. you might be deficient are the lower leaves yellowing from the tips inwards and dieing, is it climbing up the plant, if so then its N deficiency

check the site for leaf spot disease and hope it isn't that

My Sativa looking plants are starting to yellow at the bottoms also. I am feeding it high P guano, with moderate K and still a little N. The leaves are ever-so-slightly turning yellow slowly. Not from the tips, just the whole "tint" basically.
I think it is just the time of the year for me, but can't say for you unless you post a few pics.

Post pics like the others are saying but it's normal. I am pulling about 10 or so fans a day but the smaller leaves and buds are getting larger.