I kind of feel like the yellowed is happening really fast but it is pretty close to harvest : )!
Naturally dude, this is the plants big last push to old age and harvest. Its swelling and tying up loose ends, finishing calyx and seed development, hormones changing and funky other stuff plus people often tail of the nutes up to this stage so its starting to get deficient anyway. Bad reason to be this yelow and problematic is that like any nute deficiency it will agrivate a plant very quickly and just when you need that final swell and resin you deprive the plant of what it needs. Some small deficiency is to be accepted and merely a sign that your only just giving what it needs near the end of flowering rather than forcing ferts down its throat during the start and middle of flowering.
Do u know what kind of deficiency it might be?
No way dude, thats to complicated to call but i do know that at the end of flowering the plant draws on many micro and trace nutrients such as boron and manganese etc etc so a good micro or trace fert should help as well as possibly a balanced NPK i.e. 4:4:4 or 7-7-7. That is if you want to mess around with ferts, your plant and so close to harvest its your call dude. I give mine a seaweed fert at last few weeks of flowering as i also get a few trace deficiencies right before harvest.
Should think you'll make it either way. peace