Yes there is a severe lack of Nitrogen... but 36 days in... that plant will finish. Some amount of yellowing during flower is to be expected. Yours is more severe than I typically would want to see even as late as harvest day.
If you are growing in soil, one way to prevent that is to mix a low level of an organic N source into the soil for the final transplant, such as some blood meal applied at about oh I'd say just a couple tablespoons per gallon of soil. I used to add fish emulsion to my soil at 2 tbsps concentrate per 3 gallon pail.
Keep it up; that plant looks like a respectable yield is going to come off her.