Heavy yellowing at onset! Help

Is it normal for plants to drop a set or two of bottom leaves when 12/12 is induced? because all my plants do it. It(stars in the bottom leaves around wk 2 of flowering and continues for a week or so before stopping. Usually I lose the bottome two sets of fan leaves to yellowing. Is this normal>

when you put your plant to flower your leaves will start to yellow
starting at the bottom and progressing up....
it's natrual... leave them on there til they are like really dry then snip just before the leaf.... then the plant will suck all the juice outta that little twig

Losing leaves, can occur. Mine just got real yellow, and when i felt they were not doing much, I took them off. How far into flowering are you? And what nutes n-p-k did you use? I usually give mine some nitrogen for a few weeks into flowering. Among other nutes.