yellowing leaves

Is it normal for some of my leaves to turn yellow and die? 20 days left in flowering. could this be a spacing problem?

Yellowing and die off of older fan leaves is normal during the end stage of flowering due to the lack of Nitrogen.
Pics?

im just learning how to use this site so i havnt yet figured out how to post pics. this makes sense but i have a problem with 1 whole plant that turned all yellow, including new growth ... could this also be a nitrogen diff or maybe somethin else .. i will try to post pics

there is a pic of 1 healthy and one with yellowing leaves