Rapid yellowing of some leaves

Thanks guys!

I would still follow the half half your next grow round when you first see the buds start to form cuz anytime i switched from veg to straight flower nutes my leves would start changing early like yours are doing. With the half half your first flower feed you supplying that last real nitrogen feed. It will last in your soil longer because now the plant are wanting more p k than n. When you change straight to flower then when your girls want the n there isn't much there so they pull it straight from the leaves... still your grow looks pretty good. Add just touch of your veg nutes to your next feed. See if it helps. Cheers.

It's an auto, so I didn't do any switching - it's on 20/4 all the time. I started giving it flower nutes around week 2 of flowering, when buds started forming already and it was done according to the manufacturer's manual. It's been fine using them for the past 2 weeks but only recently some major discoloring started happening. I've added new picture from 15 minutes ago...

Are they actually in their 5th week of 12/12? 2nd week of budding. If so then I don't see to much to worry about, but if their in the 2nd week of 12/12 I would say you switched to your flowering nutes a tad early or you switched right to flowering nutes instead of half veg half flower nutes. Ec of flower nutes should also be less than what you were feeding in veg. When you see button size buds is when you give them half and half, next feed after that switch to straight flower nutes. This way it gives you that bit of nitrogen it still needs in the first few weeks of flower. Happy growing