Now I am on my computer with a 32 inch screen instead of my phone, I can see more details. I think you actually have nitrogen toxicity. Your leaves are very dark and starting to claw. The yellowing is what happens next. Your bacteria have probably started to get into top gear now and its too much for them. Some strains like high N some dont.
I have a G99 with exactly the same problem atm. I made up some soil, and it was too hot for 3 out of my 5 different strains I potted up atm.
If you google Cannabis Nitrogen toxicity pics you will see lots of similar looking plants.
If you are growing them big they will start to come back after a while, it looks bad, but mine is almost through it now, I put it into flower and when it starts to stretch and grow fast it should resolve itself soon I hope, otherwise it could affect my bud site development.