Yellow leaves

[Dablow]

Hi, this is my first grow. so any advice/tips are welcome.
The reason for my post is the rapid yellowing of leaves. I know It's suppose to happen in flowering stage but in the last couple of days It seems to rapidly spread.
I am on my 4th week into flowering.
Strain: blackjack.
Feeding schedule: fox farm big bloom every other watering.
Not sure if it's nitrogen diffeciancy or if it's normal for the 4th week
Here is some pics.

 

[AnimaSola]

Hard to tell with the grow light on, are the petioles red or purple or do you have purple streaks on your stems? Try posting some pics in regular light, I'm betting on CA or MG deficiency if you aren't using calmag and just big bloom.

 

[AnimaSola]

I feed mine veg nutes and calmag until they are 2 or 3 weeks into flower when the flower stretch stops and they seem to stay green longer. A handful of garden gypsum pellets sprinkled on top of your dirt before watering will add calcium without affecting your pH. A few teaspoons of Epsom salts applied the same way will add magnesium.

 

[Dablow]

Thanks for the reply, I was giving them fox farm grow big till week 2 then reduced it to minimum by week 3 and only bloom by week 4!.
Here is some pics under normal light.

 

[AnimaSola]

I do see some red streaks on the stem and red petioles, check out this chart.