Does my diagnosis sound correct??

I have a 4 1/2 week old AK47 autoflower growing in Miracle gro Organic Choice soil... I was gone for 3 days last weekend, and when I came back my two bottom leaves (the 2 bigger leaves of the 4 original leaves the plant grows) were yellow... now the bottom set of 5-pedal leaves are beginning to droop and turn yellow.. After research I found this is probably a nitrogen deficiency... My temp. is sitting at a solid 75 so I dont think it's over heating... So, yesterday when I watered I gave it water with 1/3 FF big bloom nutes.. Think this will help? if not what should I do??

bottom leaves tend to fall off, from reasons of they dont get as much light. pictures would probably help with diagnosing

Like I was about to say,
bottom leaves usually fall of 'cause of no light.